750L motor is the same as M but with locknuts (and slightly different intake ports). Yours appears to be an L then.
Intake 0,10 - 0,15mm, exhaust 0,18 - 0,23mm.
if those plates are thin (1 or 1,5mm) you can leave them out.
They look like restricors to me, but according to the part diagram from 1986 they are for all markets, usually only a few markets get a restricted version.
for replacing a drive shaft seal yes. But for the shift shaft seal the cases do not need parting. Remove the clutch, pull out the shaft and the seal can be popped out from the inside.
I have replaced many seals using self tappers and it works fine.
Hi and welcome to OSS Yann.
Didn't MIG go tits up some 10 years ago?
http://www.moto-net.com/actualites-motos-458-Business-DOMMAGES-COLLAT%C3%89RAUX-MIG-met-la-cle-sous-la-porte.html
haven't done it myself on an oilcooled engine (yet) but I'd guess you should take care not to push 26 too far in because it could disengage from 6 or the spring 27 could unhook from 28.