I'll also look into the Selphie being absent thing you mentioned.
Yeah, that's a weird thing that's bugged me before in the base game. Selphie becomes unavailable to add to the party from the time that you get full control of the mobile Garden until you enter Balamb when it's under Galbadian occupation. Quistis says, "And, ah... Will you let Selphie rest a while? I think she's exhausted from the missile base mission. She could use some rest."
The weird part about it is, the amount of time between "getting full control of the mobile Garden" and "entering Balamb when it's under Galbadian occupation" could either be laughably short (I think it's literally a straight line from FH to Balamb and the game outright tells you to go there) or extremely long (if you do all of the sidequests available to you at this point: Centra Ruins; Chocobo Forests; Shumi Village; Winhill; might be more, but those are the biggies). And you can even enter Balamb, do the (developer-intended) trick to get out of Balamb by having the kid distract the guard blocking the entrance, and guess what? You can use Selphie again when you escape! I guess all she needed was a nap on the ride over or something.
The way that I always "solve" the problem of not having Selphie available is by just clearing the occupied Balamb event before I do any Disc 2 sidequests. I want to avoid doing anything until I get Pandamona from the Fujin, anyway; that way, I can grind for abilities from Cactaurs with all of the available GFs that I can get at that point.
If I could wave my magic modding wand, I would probably just let Selphie be available after getting the mobile Garden instead of unavailable. It would be one of those nice QoL touches, in my opinion. Even then, though, I would still want to get Pandamona first for grinding reasons, as mentioned above.