Even Ironman games can be played in MP although I don't know if achievements are disabled.

You can't join an SP game however all the host has to do is save the game and relaunch it as an MP game. Hot join works which allows players to join an MP game midway through. The host (and other players I think) can save the game at any time and resume the game. The speed is often reduced during major wars to give everyone enough time to micro. Games are usually played on normal or fast. Other players can also pause however it's generally considered bad manners to pause unless something extremely critical is happening. The host adjusts the speed and can pause the game. As a general rule don't be a dick to the host as he can kick you. EG if a friend asks you to jump in to help him beat back an end game disaster he probably won't be too impressed if you immediately back stab him. This is usually decided by the host and some people will have rules/expectations of what you can/can't do to other players and what happens if you get eliminated. People will either play competitively or co-operatively. The list doesn't have any information about the empires so it's a good idea to ask the host to recommend an empire otherwise you risk starting as a 1 planet vassal of a much larger empire. Doesn't matter) then you select which empire you're playing as from a list. If the host is resuming a saved game (can be a saved multiplayer or single player game. If the host is starting a new game all players get a chance to design their own empires.

Usually through steam invites but there is a lobby where you can search for games (if I see a friend playing Stellaris I usually just ask if I can join).

How to play stellaris on lan