I had been assuming that the two grade crossings at Capen St and Central Ave would be too expensive to grade separate to make such a Red Line extension worthwhile, but reading that blog post has left me not at all convinced that those grade separations would be too expensive.
I think the assumption in that blog post that a terminal station needs to be complex and expensive is not necessarily right in this case. I think a simple island platform at Mattapan with no tail tracks beyond the platform ought to be fine: if something blocks the southbound track for, say, 15-30 minutes somewhere between JFK/UMass and Ashmont, and then all of the trains start heading toward Ashmont at faster than 6 minute headways when the track becomes unblocked, Mattapan doesn't need to be able to handle the tight headways, because at that point half the trains can terminate at Ashmont / Codman Yard. It might be appropriate to make the length of the Mattapan platform be equal to the length of a 6 car Red Line train, plus the distance a new Red Line train is expected to take to decelerate from 10 MPH to stopped, plus a small safety factor, plus maybe a fudge factor if there's imprecision in where the signal system transitions from telling the train it is allowed to go 10 MPH to the point where the signal system tells the train it needs to decelerate to 0 MPH. (The goal being that if you reach that transition point at just above 0 MPH, you'll end up near the east end of the platform, and if you hit that transition point at 10 MPH, you'll end up near the west end of the platform.)
It might be appropriate to build the Mattapan platform 40' to 60' wide, with space for ramps in the middle. At the west end, have a ramp going from the platform down to street level; if Red Line platforms are 48" above the ground (I don't know for sure if that's the case or not), then that would be 48' plus 5' for a landing (or maybe a slightly longer landing) plus probably a bit more for a fudge factor so that it can be built a bit less steep than 12:1 in case the construction is imprecise so that after it gets built slightly wrong it's still less steep than 12:1. It might make sense to build this so that if a train ends up at the very westernmost 40' of the platform, and you get out at the front most door or possibly even the next door, you won't be able to walk directly across to a train at the other side (if it manages to also platform equally far west), but you'd instead have to walk over to the eastern end of the ramp first.
If the bridge across the Neponset just to the east of Mattapan Station is currently deficient, it would probably be best to work out the ideal geometry for the new Mattapan Station and that bridge before rebuilding the bridge.
I'm wondering if a ramp from the Mattapan platform up to the Neponset Trail bridge would be practical. If a 10' elevation difference turns out to be enough to get from the floor elevation of a Red Line car to the height where a bridge can cross over it, that's 120' plus 3 x 5' landings plus fudge factors, or 135' plus fudge factors, or probably roughly the length of two Red Line cars. It might make sense to have ramp + elevator here, preferably with a fare gate configuration on the upper level so that one bank of fare gates provides access to both the elevator and ramp.
I think the blog post's list of things needed may have omitted the fare gates on the outbound platform at Ashmont; last time I was there there were none.
I'm wondering if it would be practical to build an island platform which would have its eastern end maybe 150' or so west of the Valley Road access path (which apparently doesn't actually connect to Valley Road) with a ramp down to the platform, and then at the western end of that platform, a ramp down to track level, and then an underpass to get to the south side of the tracks once the Capen St grade separation climbs high enough to provide adequate clearance.
I'm also wondering if a station could be built with the southern end of its platform(s) near the existing Cedar Grove station and its northern end near Gallivan Blvd to better maintain service for more of the existing walk shed.