G'day

1) Yeah Chunin Shikamaru with skillbreaks is a very strong support not just for you but your teammates who have Secret Jutsu ninjas as well. A must-have for onds if you want to run tanky teams imo. Danzo is also not a bad idea for a barrier ninja but maybe normal skillbroken Hashirama might be better for that (in case you picked him up for free from Fireworks)

2) I'd use them all for Cave Keys. Nagato is a very good ninja to get but not really for Susano Shisui if you ask me.

3) From past event everyone should have him 4 stars already and whether Oasis is gonna put even more frags out there for everyone I have no idea, I wouldn't count on it. Also I dunno if he shows up in any other events but you don't have to get him I think though he does work quite well.

4) Anbu Itachi is pretty good support for Shisui too I guess, no idea if it's worth to get him though since I dunno what other ninjas you already have.

5) I'd be suprised if that works since for other bond skills like Edo Minato's bond skill for the barrier you also need the Edo version of the other Hokage. Then again you c*e Orochimaru's bond skill on two of his version so if it does work for Itachi-Shisui link someone else can mention it here.