Answers:
1) the file had the alias AR used when he was scamming her boss while her boss thought he was scamming AR. there has to be more with this storyline going forward, otherwise her lying about the key makes zero sense.
2) the riot couldn't have been Castro related - way too early, plus Castro would have been maybe 5 or 6 at the time. I doubt the riots were supposed to be more than an opportunity for Meyer to let Nucky know he was also in Cuba. the only other possibility, if the Cuba storyline plays out long-term in this final season, is that it's some kind of prelude to the Batista coup that's a couple of years down the road. but even that seems unlikely.
3) the implication is that Meyer was there to kill Nucky, or at least that's what Nucky thinks.
4) the singing, dented head Milton asked him if he knew how they put the voice inside the little box of the telephone. Chalky, naturally looked at him like he was batshit crazy, and told him "electricity." then Milton wanted to know if Chalky knew how/had ever used a phone, and when Chalky said yes told him that he had money stashed someplace and would split it with Chalky if he used the phone. I guess because calling someone was an integral part of getting the cash and Milton didn't know how to used one.
5) I thought it sounded like her, but I think it was the actress playing Nucky's mom.