Nec protagonist, still in Chapter 3, have rescued Pasha and forged Memory of the Apprenti, but the old one doesn't appear aftering resting in the sphere. Is wearing scholastic cloak a necessary condition to trigger the old one?

I haven't looked at the game files for a long time now, but I am pretty sure that at least some of these things are true: you need to do all the original stronghold stuff and you need to be post-underdark for the new storylines to unfold. The content is too challenging anyway for a pre-spellhold party to handle.

Thanks. My guess is that one need to return from underdark to trigger the second half of the NEC quest. By the way, I left Athkatla for sailing at 3.91 million exp, with only Orcus unfinished. It seems to me a melee-oriented team led by NEC(+Minsc+Jaheria+Rasaad+Valygar+Aerie) is strong enough to defeat most enemies in Chapter 3. And once NEC reaches 3.4 million exp, even the twisted rune battle is easy to win.

Well, these numbers seem a bit low to me, no? And can you even reach Twisted Rune at these stages? It's been ages since I've touched the game, but IIRC you need some special rogue stone to enter the dungeon, no?

An irrelevant question: where can I find armor of stamina+5 and red star garnet? Is there any additional dropping condition?

IIRC armor of stamina drops in act 4 if you have a swashbuckler in party (unless I've changed in one of my patches). Not sure about the garnet though. What is it used for?

I found that a dual class swashbuckler can equip both enhanced armour of stamina+6 and improvised blend, which are supposed to be equipped only by a single class swahbuckler, at least in version 6.4. It tempted me to conceive an evil npc party led by lv9 swashbuckler dual fighter Charname.

Well, it's pretty easy to remove those restrictions if you are somewhat familiar with how IE game resources are handled. Look up the item codes in near infinity editor and then use those codes to find references inside the source scripts, update and re-install the mod.