While possible to control 6 in BG1/2 via multiplayer, are you telling me this is how you would recommend a new person play the game? Context is important--far more than your parsing things out of context to argue. One key difference highlighted for this person was NPC interaction, and you would lose a lot of that by going multiplayer.
I stand corrected on one point though--BG1 and 2 both are very non-linear, you are correct--but there are a lot of areas of BG1 that you will quickly die in early on so that restricts your choices until about Chapter 3. BG2 has a lot of choices that are all possible (after you go through Chapter 1). But it is true that the choice is yours--very nonlinear--in both cases.
Combat, most people agree, is the forte of IWD. I think your opinion is in the minority, and frankly sounds like complaining with your hyperbole. You are exaggerating by saying the "entire map descends upon you" from alerting one enemy, but it is far more realistic than having a group of enemies all near each other that you draw one out from at a time because you are just out of sight of the other dozen. Why would they not attack as a group?
I also see you completely ignored my point in the context of the game, though--the question of IWD v. BG is not one of quality of the story, but rather how linear a story you are willing to accept. That is the single biggest difference, beyond all of the rest. People sometimes equate non-linearity and quality of story in CRPG, but that is a false equality. For some people they may be coupled, but that does not have to be true (otherwise, nobody would read novels, as they are completely linear, or watch movies because they are linear as well).