Baldur's Gate/WMG

The Main Character from Baldur's Gate is Dexter Morgan.
Let's see. Walking around and slaughtering criminals? Check. Traumatic childhood, including having watched the death of his own mother? Check. Close relationship to his half-sister? Check. Stuck between at least two competing love interests? Check. Dead foster father / mentor figure? Check. Mass-murdering evil half-brother, killed by him? Check. An evil being inside of him, taking possession of him from time to time? Check. So, the Main Character from Baldur's Gate is Dexter Morgan.
 * Wait what? Giving birth to a Bhaalspawn is lethal to the mother, it comes up several times in ToB. By all accounts the player character had a peaceful childhood, what with being raised in Candlekeep, which is one of the nicer parts of The Realms. And the Player Character has more than one Mass-murdering evil half-brother. In fact, he's got five of them that are significantly worse than Sarevok.
 * Giving birth to a Bhaalspawn isn't fatal. The quasi-flashback shown to you in Throne of Bhaal revealed that the main character's mother, Aliana, was alive at the same that the main character was a very young child. She planned to slaughter him/her. Gorion saved the main character while killing Aliana.
 * Strictly speaking, only two half-brothers we can call "worse". Gromnir is equally bad at worst, Balthazar is secretly a decent guy, and both Sendai and Illisera are half-sisters. Only Abazigal and Yaga-Shura have really outmatched Sarevok, and then, mostly because they didn't have Gorion's Ward breathing down their necks in their starting zones.

There were only 20 Bhaalspawn, total.
"The Lord of Murder shall perish, but in his doom he shall spawn a score of mortal progeny, chaos will be sown by their passage."

Since score means twenty, this guess is assuming that it was used in the literal sense.

Only a few people in Candlekeep were killed by the dopplegangers, Not everyone.

 * A theory of mine that knocks around in my head every time I play the first game. Well, first of all, when you run into the dopplegangers masquarading as the people of Candlekeep they are all located in the Candlekeep catacombs and drawing from that, doesn't that mean that they were just about to work their way up and kill the people whose forms they took? Because of course, they will have to take those forms to avoid arousing suspicion when they go through the secret exit I.E the bookcase. And also, most of the people there are located outside of the great library so it would be really hard to get them in the first place and to top it off, the dooplegangers ALSO masqurade as Gorion and Elminster and because of gameplay you know they are not killed by the dopplegangers, so what about the others then?
 * That's what I'd always assumed; that the catacombs were where the dopplegangers waited for the couple who had already infiltrated Candlekeep to dispose of the person they'd be imitating and give them a chance to slip in unnoticed. I was surprised when I found out people thought otherwise; just the way they talked made it seem clear to me that they hadn't had a chance to get a good read of the people they were meant to eventually imitate just yet.
 * And because they are acting out of character attempting to imitate them yet you run into the people they are trying to imitate i.e Winthrop, Dreppin, Reever etc outside acting just like themselves, I can only assume that by killing the dopplegangers in the catacombs and just outside in the libary itself you inadvertently ended up saving their lives, and thus it acts as a subversion to a player punch.

"Boo" is a schizophrenia/split personality of Minsc.
By all rights, Minsc should not be able to cast spells with Intelligence and Wisdom scores as low as his; indeed, there's a question of whether 2e would let him stay a ranger. However, given how sensible Boo's advice tends to be, there's a possibility that Minsc used to have some measure of sense...before his head wound. His head wound ended up slightly jarring the communication between the two spheres of his brain, giving him a case of what some psychologists (usually fringe, admittedly) would call a "bicameral mind". The spheres are communicating, all right, but the perceiving side isn't quite realizing where the reasoning side's messages are coming from, so it projects them to the most likely candidate. In this case, a certain hamster. When viewed in split personality terms, Minsc is a personality that's always forward, but manages to stay in some measure of contact with the other personality, to the point that the Boo personality can even use Minsc as a conduit for spells. Minsc still has the spell-casting requirements; they're just squirreled away in the Boo component.

Imoen’s Bhaal-essence is “different” because it attuned itself to that of the main character over their long association.
''“We're dead, aren't we? If I'm a Child of Bhaal, shouldn't I be... be gone, then? Just like Sarevok? Irenicus said I was different, but... I don't know. But I know I'll follow you wherever I can.” - Imoen''

Instead of her essence/taint being sucked into the black hole that is Bhaal when she dies, facilitating the typical Bhaalspawn disintegration, it stays in “orbit” around the main character until he either resurrects her or dies himself. This additionally serves as an explanation for why she’ll stay by Charname through thick and thin, even if he’s a mass murdering psychopath completely at odds with her own alignment.