make sure to go with a very thin PCB. The original in the pics is much thinner than most PCBs, and you'll very much run into clearance issues with keycaps if your PCB is too tall.
I'd be down for a KB's worth! I think this'll much more evenly light a keycap.
Bytheway, I think you usually pick the LED and ship a roll of them (usually 4000 or so) to the fab who will then combine them with the PCB you have. Some places will have common parts on hand, but IDK if they'll have SMD LEDs in that size.
as to fitting inside the switches: could be done, but I'm not sure why you'd want to. It'd make it awfully hard to close, especially with brittle cleartops (great idea with the white top though: also acts as a diffuser.) Not sure about clearance of LED and the switch leaves on the "back" of the MX switch though. You could maybe do one LED on the front and 2 on the sides.
You should look into thin PCBs and what fabs offer them. I looked into this a little when I was looking into getting a flex PCB done for an M with NKRO, but sadly I needed PCB files to get a quote from anyone.