Cotter picked Welsh over pot plant Cusack because he wanted more around the ground from his backup tighthead, the same reason Grant is playing ahead of Gordie Reid at loosehead. Welsh however now seems to be huffing and puffing after 20 minutes, so that's probably a mistake in the squad.
Starters are good enough to match the Bokke, both at set piece and around the ground, but the reserve ones might struggle a lot.
Re: Russell, he's just fantastic. I genuinely think that whatever happens in this world cup beyond this point (as long as we secure third) will be a bonus. More or less all of the squad except Ford, Strokosch, Laidlaw and Lamont are young enough that they should still be playing to a high standard for Japan 2019. Outside of that, McInally and Dunbar missed the world Cup through injury, Fagerson, Hoyland and Toolis are fabulous players at 19, 19 and 21 respectively who should only grow in ability, and then there will be a handful of imports to add depth and competitive within the squad, assuming they don't commit to someone else. Nasi Manu, Anton Bresler, a Glasgow boy who's name escapes me.
If everything goes to plan, we should have a very very good team come Japan. We were one of the youngest and most inexperienced sides in the cup (not withstanding Fords 80 caps, Laidlaws 40, Strokoschs 40 and Lamonts 98 odd), so we have a lot of time to develop both individuals and combinations.