*Snicker!*

I don't remember Jaheira and Khalid leaving if the party attacks Seniyad in Cloakwood forest. Does this happen in the original Baldur's Gate?

No, only modded.

I just re-installed the original story and I must say I prefer the experience to the enhanced version, all other aspects being considered. The tighter focus that the 640×480 pixel plane grants the player is a much better way to see the game; I found it pulled me in to the story better. (Though, if one had previously beaten the game, I can see how a refresh of the team might add some zest to the replay.)

A notable con is that the original game has a quirk about pausing; don't mess with your inventory after casting a buff spell otherwise you'll lose it. This means that, once you have decided to tackle a map, the best advice is to pause as you tactically position your team to take the next area, only, and then repeat until either out of healing or the map is complete.