Any question answered...

I was thinking of a downtube lever, they usually have a bit more spare capacity.

If it is, you can shim it, but in practice the cages aren't usually so tight that the extra 1.3mm is a deal breaker.

Am I right in thinking that using a 8-speed friction shift lever for a sram 10spd rear mech will not work?

something to do with the pull ratio.