I think multiplayer is always hysterically unrealistic no matter what you do, because people in one-off matches have incentives that nobody but a PCP addict serial killer has in real life. That being said, I just wanted to comment on something you asked, "why are you breaching the building, given how dangerous it is"?
And this is where Single-player is unrealistic. We are breaching the building because we want to breach the building: run in and shoot a bunch of people. There are lots of Raven Shield levels I could have beat by creeping around and getting my objectives knocked off. I never did that. Instead I found and shot ever single enemy. Being able to mass murder people in a realistic way, to execute dicey and violent procedures on hapless foes, is the entire point of the game. This is why I despised one of the last levels in the original Rainbow Six, where you have to sneak around an executive's house avoiding his butler Oddjob, and you're not allowed to kill him. If I had had a Gameshark for N64 at the time I first played it I would have skipped that level.
I, personally, play tactical shooters to be assassination/breaching sims; I loathe PvP, though, so that means it's got to be against AI in missions. This is also why you'll never catch me dead playing the new R6 and GR games; less realistic, more arcade-y and Normie-oriented, faster, and multiplayer focused. I went from buying every single title in these games to refusing to play any of them, because they basically did everything they could to make me dislike the actual experience.