No idea, because the completed Milford extension study never ended up being published. So we have no idea what track layout the branch would've had, and where the passing sidings would've gone.
Yeah...about a decade ago. That was the one that was supposed to benchmark Milford vs. Hopedale termini. It was supposedly completed, but was never published.
Alon Levy wrote about it on the 12th, so it got some traction in the transpo blogosphere before the vote. Although at that time Levy was taking Sen. Will Brownsberger to-task for foot-dragging. Amusingly, Brownsberger himself showed up in the comments section of that piece.
Sharon substation was only built with enough capacity to handle Amtrak's power loads, so that would have to be expanded commensurate with T service levels on Providence, Fairmount, and in the terminal district. As you can see from the Google Satellite view, the sub's space is nearly empty so is pre-designed for generous expansion. They'd also have to wire up Pawtucket layover. And one of the platform tracks at Attleboro Station may still be unwired (though I think there's already an ongoing item for backfilling that one simply for sake of Amtrak track-switching flex). That's requirements just for the base Providence schedule. If they want the T.F. Green + Wickford Jct. runs to be electrified as well, there's additional construction work (such as a freight gauntlet on the T.F. Green platform track so the P&W autoracks can clear 'between' wires) that would have to be programmed RIDOT's way.
There are no plans to raise the speed limit of any part of the Worcester Line to 90 MPH. That requires a large expenditure for uprating the track maintenance class from Class 4 (79 MPH) to Class 5 (90 MPH), with all associated costs for updating the signal system. It wouldn't make nearly enough difference to nearly enough trains in an area as relatively small as the Wellesley-Framingham stretch to end up paying off, so it's not being planned right now.