the Bhaal essences scattered across his progeny must be rejoined through the deaths of each Bhaalspawn, whose divine essences then return to the source seen at the end of BG1.
The in-game lore is silent on whether it is possible to resurrect Bhaalspawn after
their divine essences have rejoined the pool, though the complete crumbling of the Bhaalspawns' mortal shells after death would rule out the game's resurrection spells which require bodies or at least body parts to function properly.
This is why Sarevok needed CHARNAME to
do the resurrection some other way, and even then, it wasn't possible to bring Sarevok back with his divine essence intact as it had already entered the pool.
Imoen is different, because
the devs changed her into a Bhaalspawn for BG2 after previously envisioning her as a normal human in BG1. Because the devs viewed her as a normal human in BG1, she could be resurrected like any other normal human without her body crumbling to dust. The devs were thus faced with a difficult conundrum, because the intended shock of discovering Imoen's Bhaalspawn status in BG2 would have been nullified if they retroactively patched BG1 to make her non-resurrectable (because you would see her body crumbling to dust in BG1 and realize the two of you share a common heritage, which you then inexplicably wouldn't be able to explore or even ask her about until BG2).
This does represent a plot hole, yes, but the cause is out-of-game human error rather than inconsistent in-game lore.
Shapeshifter, jester, beast master, and wizard slayer strike me as the weakest kits in the game so some combination of them would give you a pretty weak party. In terms of joinable NPCs I'd say the game's singleclass characters tend to be the weakest:
Alora/Skie/Safana are weaker than Coran Khalid/Rasaad are weaker than Kivan/Minsc Branwen/Faldorn are weaker than Yeslick/Jaheira Xan/Xzar are weaker than Baeloth/Edwin
I'll pick Skie, Safana, Rasaad, and Khalid for all-around weakest party. (Eldoth gets an honorable mention here but I think bards are an underrated class due to their high caster level, which helps Eldoth's casting power even though his stats are poor.)
Dsimpson and Haeveron regard FMs and FMTs as the game's strongest characters and I think they're right in that assessment. This is often counterintuitive to people who haven't tried those mixes because you can't get time stop (or in the case of FM get it late) and can't wear armor, but mage buffs have such great synergies with fighters that the deficiencies are more than offset by the advantages.
Other very strong characters are cleric/rangers (though less so now that they get fewer druid spells), half-orc fighter/thieves or fighter/clerics, inquisitors, and as others have mentioned sorcerers and bards. You won't go wrong picking any of them.
There are also a couple of joinable NPCs that are worth mentioning because of the special features they have. Jaheira, for example, can not only wear heavy armor (which only FDs can do) but is also able to cast some cleric-only spells like raise dead and zone of sweet air, making her the most versatile druid character in the game. And Edwin gets, essentially, the spells per day of a sorcerer while being able to access the much broader spellbook of a mage, making him the best arcane caster in the game (if you can put up with what some regard as his grating personality).
The difficulty levels exist because the devs want to ensure everyone can enjoy the game's storyline. Not everyone who struggles with normal difficulty level is a "bad" player; some people just don't care enough about combat to devote much time to mastering it, while others just haven't had enough experience with the game yet. It's also worth mentioning that, like Pillars of Eternity, BG1 doesn't do much to shield low-level players from difficult encounters, as the ankhegs (or Pillars' cave encounter) can attest.