With a premade party you'll miss quite a bit: all the party-member related quests, interactions and a lot of immersion.

The majority of quests and areas are accessible though, so I wouldn't worry about that.

They are not vital, but you will miss quests from (in no particular order): Jaheira, Keldorn, Mazzy, Valygar, Nalia, Anomen, Cernd, Jan, Korgan. I don't think I've missed anybody. Most quests, with the exception of Jaheira, are quite short. Romances are not an issue if you don't intend to have any Bioware npcs. No areas, except certain npc residences, are inaccessible, but some will be missing content.

Thank you, that's the main thing I wanted to be sure about.

If you're a real completionist, you'll want to consider modding your game so your character can get all the strongholds and their related quests. Otherwise, you're going to have no choice but to miss content. However, I wouldn't recommend that until you've played vanilla at least once.

You could have a party totally made by you, but as others have said, you would miss out on a lot of the immersion. I personally think that having characters with weaknesses make the game more interesting. You can't simply power through everything, you need to be thoughtful in your approach to combat.

With that being said I played one game with two player made characters. My main character, a ranger I think, and then a Mage/Cleric that was totally maxed out. You could consider something like that if you wanted the power gaming aspect as well as the flavor that NPC's bring.

I played both BG1 and BG2 with a full pregenerated party, I like it that way. I don't know for BG1 but in BG2 I could dismiss one of my own character, enlist one NPC and do his quest (did it at least for Valygar and Korgan) then dismiss him and reenlist my previous character.
I think most of the NPC linked quest can be done this way.

Did you play with addons? From what I heard (I might be wrong) if you dismiss pre-generated character he will be lost forever. Will that character just stay where you left him? I didn't check it myself though (my attention is taken away by other games currently).

I played without addons and a long time ago, the dismissed character just stayed where I left him. I only did it in BG2, didn't try in BG1.

The real loss is romancing. Kiss your waifu goodbye. Also, you have to suffer Jaheira's presence, at least up until hell fight with irenicus. Without her, you cannot access the Harpers' Hold, obtain some unique mage scrolls (one, i think?), which cannot be found elsewhere, and loose some nice pieces of story like the Harpers turning would-be-assassins on your sorry ash, and making Jaheira wanted, despite her and Gorion having been renowned organization members. Another real loss is Sarevok. The man is a beast concerning stats. Equip him with a ToB plate giving +1 dexterity, and i seriously doubt you can roll anything close to or better to him. And of course, the character backstories and ending text, which is not a real loss. Remember:

1) Jaheira only until you complete her quests must be in party
2) Take up Sarevok. Maxing him means grandmastery in Two Handed Swords, Crossbows, and (i picked) Halberds.
3) Take up at least one romanceable npc. If you are female, only Anomen can be romanced. In reverse, Aerie, Jaheira, Viconia.

You can still create 3-4 characters, and fill the remaining slots with Imoen (sister), woman (romance) and sarevok (brother). Creating 3-4 entities of your own, is more than enough; if made properly, they are even more crafty than an entire party of six! This way you mix finely, with balance, between RP goodness 2 offers, and self made self controlled powergaming.

The real downside now of recruitable npcs, is their HORRIBLE weapon proficiencies! Not even close to what i really needed to pick.

Even if you don't count the exclusive nature of strongholds (unless you use a mod), is it actually possible to experience EVERYTHING in BG1-2 in a single play session? And even if it's theoretically possible, wouldn't you have to look at some spoiler-y information to sort of plan out certain quests ahead of time (so that you can do them before you lose access to the conditions needed to do them)?

Indeed, theoretically yes. But without a walkthrough filled with spoilers, you won't see everything.

Due to the unfortunate shortage of evil party members in BG II, I was considering making an evil character or two to add to the party so I wouldn't have to deal with party members leaving/attacking me when I do evil things. I'm hearing that this will cut me off from some content here. Would it be possible to build a party with a PC, Edwin, Viconia, Korgan, Imoen, and a premade character and still get the banter from Imoen, Edwin etc? Or does messing around with the multiplayer-singleplayer trick irreversibly mess stuff up? I'm also curious how having another premade character in the party would impact the story/cutscene. will it randomly determine which character is the bhaalspawn when plot stuff happens? Will one character show up in the dream sequences one time and the other another time-that sort of immersion breaking stuff?

First up - you don't *need* 6 party members - I played evil and just had PC,Edwin,Viconia and Korgan and it was a strong party (PC was a thief though) - you can take Imoen or Jan along with the above party if you're not a thief.

In terms of 'Who's the Bhaalspawn?' - it's the first character you have before you import the others - any others imported into single-player from multi-player will be just be banterless NPCs.
But no, it won't mess anything up.

You missed Minsc