Actually I'm not deciding it myself, I'm following PnP, and I was even quite "forgiving" for player's sake because else I should have removed from druids quite a lot more spells (e.g. Doom, Bless, Cloak of Fear, etc.), and clerics should have the incredibly more restrictive sphere system used by Divine Remix.
Only Morninglords of Lathander should be able to cast from the sun sphere, and druids were never able to cast the Cause Wound serie of spells. What's CS by the way?
P.S Cleric should be able to handle undead monsters with their innate ability to Turn Undead (if not, what's there for?), and already have a bunch of anti-undead spells.
Actually it's not a matter of divine spells being counterparts of arcane ones (not to mention gate is another arcane spell thus I don't see much difference in that regard), it's that, as you noticed, summoned demons are extremely powerful, and letting a L14 priest summon a 24HD Pit Fiend would be outstandingly OP.
In theory I could restore Gate, but it would still summon weaker creatures than its 9th lvl counterpart for obvious reasons. I even though about making it more like PnP, using good aligned creatures too for non-evil characters (e.g. Devas), but that's another story.
Last but not least, I really don't think priests should be better summoners than mages (the latters even have a specialized kit, the Conjurer), there's absolutely no reason behind it.
I'm sympathetic to this, but I don't think it will happen, sorry.
Heavily modified or replaced spells
When it comes to compatibility I assure you there's no such issue, but I partially agree with you. Anyway this is something I very rarely do, when it happens it's for good reasons*, and I never do it without asking player's opinion.
I consider SR a community mod almost as much as a mod of mine, thus I try to discuss everything with players (which also makes much harder/slower to mod trust me), and what you see generally is the end result of long debates. Both NB and Symbol changes were heavily discussed back then, especially the former. If your wishes meet the ones of the majority of the community then it's quite probable I'll please them. Long story short, I always encourage feedback (even negative one) and feel free to open discussions on those things (like this post) but remember I can't please EVERYONE, I think it's obvious such task is almost impossible.
*Vanilla's NB was almost broken because the AI cannot cope with blindness as players, whereas Symbol of Fear was kinda weak as a 7th lvl spell considering a 1st lvl spell can both cure and protect the entire party from it (in fact it's a 6th lvl spell in PnP - as a side note I may restore it as such).
I agree, in fact this won't happen in V4, and Mages will probably get both Animate Dead and Summon Shadow.
We recently discussed this a little and within V4 quite a few summoned creatures will be slightly weaker (e.g. less hit points), but most of them are very fine in terms of concept, abilities and qualities.
I know, and I agree, though SR almost gives for granted that SCS will be installed with it. What I said above (about nerfing some summons) is particularly true for demons and celestials but don't expect them to become extremely weaker than they are now
Summoned creatures granting XP
Well, time permitting I could make it so that only AI summoned creatures grant XP, removing possible exploits. I'll take it into account when I start to seriously work on V4.
SR doesn't remove such cap, though many players asked for it, and I may be favourable to add it as a separate component.
Do summoned demons ever had a cap? I'm not sure. Anyway, SCS removes their "summoned demon" flag, thus any eventual cap, and it's not SR's "fault" if you end up fighting tons of them.
I'm sympathetic, but I'm not sure this will happen. Vanilla's items are very "outdated" in this regard, and don't use actual spells but custom effects. I could spend a lot of time to explain you what this means, but let's just say that I would have to seriously alter the itm files to make them work as they should, and that I'm not sure doing it within SR is a good thing (even if it can be done via patching code the end result would be a heavily modified item).
That being said, how many times SR spells are so much different than vanilla's ones? Not to mention you can consider spells cast from items as custom spells rather than perfect copies of the original ones, which is even more true in vanilla because it's exactly what they are and how they work.
Except a few "must have" re allocations (e.g. SCS ones are mandatory for compatibility reasons) such component will surely be separate from the main one, though, as with many other IR components, I'll always give for granted they are installed for various reasons.
Alterations in the spell system is what concerns me most as I doubt the AI can use their new spell book efficiently. Maybe SCS can deal with that but what will happen if SCS is not installed? I have two copies of BG installed, one with SCS and one without.
The summoning cap -
I had no other spell tweaks installed. No summon cap removal or celestial cap removal.
I found this not long after I had started a new BG2 game when I used Monster Summoning I in the starting dungeon. I was able to have more than 5 hobgoblins under control. Then I tested with those celestial summoning spells and found that celestial creatures was no longer limited to 1.