10 to 11 Speed - Hub, derailleur and Gearing Question

[pdonk]

When I switched from 11 speed shimano to 12 speed sram axs, I ended up with an "extra" 11 speed ultegra groupset.

The bike I want to put it onto has an 8/9 10 speed chris king classic hub. As I understand it the option I have is to get a 34 tooth mtb cassette as it should fit over the flange on the hub.

Out of curiosity, I tried putting the 11 speed cassette on the hub, with one less gear and with the 1.85mm spacer. It fits snuggly, with no wiggle between the cogs.

Questions:

1) Will a SS derailleur work with a 34 tooth cog and 34 tooth chain ring, if not will a wolf tooth link make it work, or should I just get the GS derailleur or an 810 derailleur?
2) As 10 cogs and spacers fit properly, is it ok to just use it as a 10 speed? It is a middle cog that is removed? Any issue running it as a 10 speed, and locking out the derailleur so it only shifts so far?
3) Is a jump from 15 to 19 teeth too big or should I go 17 to 21? Both are 4 tooth jumps, but wondering which is a better jump.