1 : Get Mordred first, then go for Arthur. Arthur just gives you a little more damage. Mordred on the other hand, gives you overdrive control, and B frame debuffs if you lack them.

2 : Both are quite bad. Exchange shop eidolons are quite useless when you can get SSR eidolons during events. And shop weapons have bad stats, even compared to disaster weapons.

3 : Yeah, you should be able to do them (it kinda depends on your team tho).