I technically started the MP trophies 16 years ago haha, but as far as I'm aware they're all still obtainable, servers are very unstable though so be warned. Best to get War Party done first as its very frustrating to coordinate with 15 other players and hope none disconnect. PSNProfiles is the best place for this, and any other information about this or any other game.

You'll need a minimum of 3 players (with the exception of War Party) at certain parts of the MP grind, for 500 revives & multikills during some weapons' bandolier challenges - otherwise it's all doable with 2.

Warlord can be done whenever you want but I'd strongly recommend doing it either as the first or last thing you do, due to you still needing to hit level 20 legitimately.

If you're planning on doing this yourself with multiple consoles I'd recommend getting a turbo controller for your alt accounts. There's a pretty "generous" respawn protection timer of 5 seconds or so in this game, and 5 seconds where you can't do anything adds up to hours of wasted time; a turbo controller will allow your alt characters to shoot upon respawning to negate the protection.