I love your interpretation of how things were with Zell, Squall, and Irvine. Here comes this interloper edging into this bromance and trying to force an OT3, the nerve! Irvine had basically set the tone of their friendship from the start, his tryhard behavior did not leave a great first impression. He still remembered Zell from their super best friend little kid days, it is hella awkward and unnerving when a stranger comes out of the blue acting like they know you and you have no clue who they are. (I've experienced this first hand when I moved in with my twin after I left Ft Jackson. I found out she had a girlfriend because of that happening.)
What you're saying makes perfect sense! I think over time Irvine has earned the friendship with Zell he's doggedly chased after, but they tend to butt heads because they deal with things so differently. Also, now that Zell is a dad, I think Irvine tends to be that annoying uncle that comes over uninvited an hour before bedtime with new awesome toys and a ton of candy but never sticks around to help put the super excited and full of sugar little girl to bed.
Red is exempt from Irvine's advances, not only due to the age limit but also because she is with Edgar. He's cheering REDGAR from the sidelines, yo!
Al-Cid would get free upgrades while he's a cadet, but he'll take the charming smiles and winks as tips. He finds Al-Cid amusing, it is rare to meet someone who is as much of a flirt as himself, probably even more so.
I really like the idea that Irvine and Elena might know each other from their school days. Due to Doomtrain's residence in his head, the details on how well he knew might be fuzzy, but his constant attempts to seduce her leading with 'You and I, we've met before...' though it usually follows with 'in our dreams' or some other ridiculous shit. His persistence in following her when he is around, and his interest in her might cause a bit of paranoia on her part?