Welcome to Eorzea!

I'm not a crafter so I can't really answer your first question, but for the other two, firstly, this game isn't really alt-friendly, in that it's extremely difficult to send gear between different characters (they have to be on your Friend List to send items via the Moogle Delivery Service, and that's not possible with alts as a character has to be logged in to receive and accept a Friend Request), but I do know many players who do have alts for RP so it is possible! (I have alts myself but... I barely touch them sadly, I have enough on my plate playing Enki enough as it is!).

For your final question, the current highest equipment item level available is 665 I believe, and yes, this is further raised with content patches and expansion packs. Best in Slot is really something that isn't a major thing to worry about here, because the gear level is being raised frequently, what is current 'BiS' quickly becomes irrelevant, and eventually, junk useful only for glamour (and fights become less about what gear you have and more the skill of the player). And unless you're running endgame stuff like Unreal/Ultimate fights, it's not needed. As long as you're keeping up to date with gear before the level cap (currently 90), then it's not a big thing to worry about.

Anyway, welcome again!