Returning Player - Cross Realm / Cross Faction Guilds

I moved abroad during DF and made some new characters on oceanic realms that I played casually. I have now moved back to the US and want to play with some of my old friends. I played alliance characters on frostmourne, but my friends have guilds on horde, mal'ganis and horde, area 52. I need to transfer back to a US datacenter server because the ping is too high.

How exactly do guilds work now?

Will I need to transfer to their server and realm or can I transfer to a lower population server to get the names I want?

Can I remain alliance if they are horde and still be in the same guild and raid (mythic content too)?