amazing how much difference platforms make, found with mine (Metcalfe N gauge kits) it made a huge difference. even with the rest of the layout being utter pants for reliability it looked better
I'm running N scale and I was considering making my station platforms from balsa wood, both my platforms are curved. BUT then I discovered Peco NB-26 Platform Edging - Brick type. Thank you Peco 👍
Rather than a ramp, you could just put a set of steps at one platform end, maybe the other could terminate with a metal rail. The main reason to include ramps is to allow track level crossing, but you have a subway, so carts can cross platforms that way, if you have a ramp at each end of the subway! You could skip modelling the subway on the nearside platform by hiding it under a station concourse - waiting rooms, cafe, news stand, WH Smiths! You could include a lineside hut at the bay end of the platform. And maybe a solid buffer-stop on the platform at the end of the bay, with a sand-trap to stop runaways.
As for your binding issue with the Mk3 coaches, you could do what they've done at Birmingham New Street at the crossovers - shave a little bit of the coping stones on the platform edges.
Regarding your problem of platform clearance from your Mk 3 coaches, instead of shaving your platforms down why don't you just pull your track over 1 or 2 mm. Also re your rough jigsaw cutting problem it is because jigsaw blades cut on the up stroke, you can get blades that cut on the down stroke like most saws do, which will give you a neater cut.