Can gold from the Gold Capture ability be used to build a fort?
No. The captured crew gains a gold value, but doesn’t become treasure. Forts need (non-unique) treasure specifically.
If a player uses the Altar of the Loa UT to force an opponent’s ship to explore an island and the Castaway UT is found, I assume that the Altar user chooses which crew is brought into the game based on this line from the Code? (because the Castaway ability only even happens because of the explore action itself)
Even though the explore action is what causes it to be revealed, Castaway’s ability still falls under the same category as other abilities where the owner gets to pick what they get, similar to choosing what they lose if their ship loses a boarding action while under the effect.
The part about the Altar user getting to choose applies to things like a Captain, where that player gets to pick the targets, or whether or not to use the ability of a face up Helmsman.
(0LR+5 discussion)
It’s mostly going to depend on the group, but like Ben said, players generally just build a 65 (or whatever +5) point fleet and toss the extras in right away without thinking about it much. It gets a little trickier when players purposely set aside a few different options ahead of time depending on what they might see in the opposing fleet(s), but even then, the choice of which specific option to pick is usually pretty obvious, so having everyone do that phase simultaneously is going to work perfectly fine the vast majority of the time.
Doing it in turn order is only necessary if players get into one of those annoying “If you add that, I’m going to add this!” arguments, or when you need a highly-rigid timing structure for a tournament game or something along those lines.
If a player has both the Hoard ability and “eliminate all of the other ship’s crew” boarding Massacre ability on the same ship and wins a boarding party, can they steal all treasure and eliminate all the enemy crew? (this is distinct from the RtSS Headhunter, who gives an either/or choice)
Unless something else prevents one or the other, you can use both abilities in full during a single boarding.