^ Great questions. I'm not an expert, so don't take this as gospel, but here's my understanding...
Blue to Riverside
Don't think Riverside Yard's necessarily a blocker. If the system's going to expand to elsewhere like absorbing the entirety of the north-half Urban Ring, you've got plenty of heavy-repair options elsewhere. Recall that back in the 1945 expansion plan they were going to dual up Riverside with an HRT line (of TBD affinity) on the B&A through Newton meeting the streetcar D at Riverside at a massive dual-mode yard complex. The yard wouldn't have been any bigger than today's. It simply would've been augmented by other sites (Watertown, Arborway, Reservoir...and GLX since that was on the same '45 map) reflecting the more cramped portioning.
For example, there's no shortage of industrial sites for fashioning a heavy-repair carhouse on the Everett/Chelsea side of the Mystic on Urban Ring NE quadrant or on a Massport parcel somewhere outbound of Silver Line Way.
The other problem, ironically, is the very extension to Needham you propose. Converting Needham Junction-Needham Heights to LRT and connecting to the Green Line at Newton Highlands has been on the books for 80 years, and has been a serious proposal for most of the last twenty years. In fact, if I had to put money on it, I would bet that Green to Needham will be the next rapid transit extension to open after the current Green Line Extension project to Union Square and Medford Hillside finishes. It's still a good ways off, but it's easily the most reasonable rail extension we ever discuss on here.
Yes...this one. The 7 grade crossings work spot-on with LRT, but would all be required eliminations with HRT. There's little compelling reason to eliminate: Upper Falls, Gould/TV Place, Needham Heights, and Needham Center stations can all flank their platforms on opposite ends of the crossings with graft-on signals for transit priority and crosswalks. The others--Webster, Rosemary, May, Oak St.'s--simply aren't bad enough for traffic on a pretty well-distributed downtown grid for going all heroic with the grade separation. The only crossing here that *may* be worth eliminating is Gould because of the burgeoning TOD up the street that's likely to sharply increase traffic volumes over the crossing (the actual 128 Pn'R lot here wouldn't factor, being fashioned on the Muzi Ford lots well south of the crossing).
The price difference is stratospheric enough to throw a monkey wrench in an otherwise straightforward conversion from Commuter Rail...which needs to happen
sooner because of the NEC SW Corridor traffic pinch's effects on RUR implementation and Amtrak growth. Put it this way: the Rail Vision is cranking this one up fast as a burning priority, so you simply won't have enough time to figure out a way to get Blue to Kenmore before the Needham Branch has already gone Green out of Rail Vision expediency. So the value proposition becomes "Why Blue
after it's already gone Green?" and trying to tally up the above-and-beyonds for the later HRT mode change. That'll be hard when Green is plenty improvement by itself.
Yeah...adjust the Eliot stop spacing if this is a thing. But I also agree this really isn't a thing. It's not like there's anywhere along 9 to put a park-and-ride. These are all thoroughly neighborhood stops.
The fact that the D stop selection is so very baked-in from 19th c. re-use of old Boston & Albany branchline stops also means you have trouble with the utilization scale-up to 6-car HRT trains (which, mind you, can get longer with higher seating capacity much closer...if not guaranteed exact...to Orange Line specs once dimension-stunting Bowdoin curve is eliminated) vs. Type 10 supertrains. The only big TOD growers are the Upper Falls/New England Business Center/TV Place trio on the Needham Branch where the capacity "increase" is getting first-time transit at all. All of the D stops are what they are, and Riverside TOD growth will be counterbalanced significantly by park-and-ride volumes diffusing more to Highland Ave. @ TV Place to the south and Fitchburg Line urban rail @ Polaroid/128 to the north. Hence, the coming-attraction parking reductions at Riverside with the full TOD buildout. So you're not really looking at ridership increases to fill a lot of those king-size HRT consists. Arguably, 6 mins. each to Riverside and Needham is going to clear out tons of crowding between Kenmore and Newton Highlands on the heaviest-use portion and fix whatever glitch may exist today. The capacity argument for conversion is weak at best because trains that large are going to run that empty through unchanging village stops.
In short...you could swallow the D to say you swallowed the D. But the relatively tame use of per-train capacity plus all the extra expense for 7 Needham grade separations begs to ask "Why?" Keep in mind, if you get BLX to Kenmore in the first place you are geographically unlimited in where you can go...including Allston/West, a kooky bend-back to Downtown, or alt-spining up Brookline Ave. There's no gun to anyone's head saying they have to have an immediate answer as to where to take it next...and "oh well the D is just sitting there". If you've got a way higher-leverage westbound extension but can't fundraise for it for another 30 years...just sit at Kenmore. The routings will always be there. Make your ultimate choice count to the max.
Put it this way...your highest-leverage touch of Huntington is going to be alt-spining the Central Subway with a dig extension under the reservation then some sort of link-up to the D in the Brookline Village vicinity. Then of course replacing malformed Copley Jct. with a Back Bay stop that ties in via Marginal Rd. (urban renewal nuke zone easier-dig) to the South End tunnel @ Boylston and choose-your-adventure routings therein. Because that's where a radial-distributed Green Line with north-half Urban Ring, Harvard Spur, Porter/Route 16, Seaport, and Nubian can all blend as cogs.
You do have potential HRT crossings...but they're crossings. Like, for example if you took that Kenmore BLX up Brookline Ave. to Longwood and diverted down Fenway + Ruggles or something. But that's different than a straight-on Huntington Subway extension, which very clearly goes in direct service of Green-as-routing-'blender'.
Still would caution against overrating BBY too much. Because if you envision that E relocation as the first step in the
eventual alt-circuiting of the Central Subway the Green side of BBY is going to one day be seeing way, way denser than 6-min. headways. With a max-cranking Orange, all manner of RUR'd Purple Line service, and hopefully NSRL taking linking Purple north I can't fathom what would still be lacking at that node meriting an HRT triple-up.
Downtown Capacity
There is a thread called Green Line Reconfiguration that goes through a lot of these ideas, but basically there are two main proposals.
Bingo. Central Subway has a higher native capacity than right now when all the signal/vehicle/dwell cruft gets solved by the ongoing GLT effort. Then it's simply adding better distributing cogs. Take for example how much less-overloaded Park St. would be if there was a one-seat via Boylston to the SS/Seaport, and Red-Blue ended the DTX/Park double-transfer. How many fewer people make the mad upstairs/downstairs scramble? How much quicker do the doors close? 15%? 20%? It might be carrying exactly as many passengers as today from other forms of growth, but it would SEEM like a Green Line station only half as busy because the cloggingest/time-consumingest of ped movements slowing the whole works down would be reshaped far away from there.
Also...since straight-on repeat of the failed Silver Line Phase III routing down Essex St. is wildly impractical and a physically difficult tie-in to the GL level vs. recycling the abandoned Tremont tunnel @ Eliot Norton Park, we probably are looking at a South End jog of some sort. (There are a couple different trajectories so your tastes may vary, but Tremont to the Marginal Rd. traffic island well-studied by the SL III Washington St. tunnel portal looks sanest). So you also have potentially rich multi-routing potential from a 4-track junction at the Pike that includes the E's new Copley Jct.-replacement hook-in. Meaning...if you had E-to-D surface trackage you could right away do a Kenmore-Design Ctr. or Kenmore-Nubian one-seat alt pattern through that junction. You can either/or any of the northern branches--Medford, Union/Porter, Urban Ring NW to Kenmore, Urban Ring NW to Harvard, Urban Ring NE to Chelsea/Logan--to Seaport, Nubian, or Huntington. And that's where the upside of eventually burying the full E to Brookline Village really shows its bona fides. Because then you have multi-directional junctions at Brookline Village, South End, Brickbottom, and BU Bridge to enormously blur services at distributed capacity that dwarfs anything we've ever seen.