I am also a long-time Trinity player, but have never tried multiplayer. Honestly, the way the experts describe the experience, it sounds like a duel with shotguns at 3 paces, and a very different game than what I am used to and enjoy. I would be willing to experiment, however. But based on my lack of knowledge, I do have a few questions, which I might as well ask here:
That's the competitive online experience. Games with friends or other people who aren't pros won't be like that. Given that most the pro scene has left to Rebellion if you can find random games online they'll probably be fine too. And you can always keep the game speeds on normal if you really don't want to encourage rushing.
01. Is it possible to embed 2 human players in an environment with AI players as well, or is it only human vs. human?
You can have AI as well. Besides the pro online games, the other main type of game are "Comp Stomps", where teams of humans take on AI together.
02. Does the game have to be played all in one sitting, or can everyone pause and walk away for a day and resume at a mutually agreed upon time?
You can save multiplayer and reload later, though obviously you need to have a fairly organized group so that you all come back at the given time.
03. If it is possible to pause and resume, can you look at the game in the meantime?
Yes, you can pause, and everyone can look around and give orders in that time. However, any other player can unpause the game, and you only have 10 pauses per match, so you should make sure all the other players are fine with you pausing before doing that.