But it requires starting multiplayer mode, which isn't always possible. (I tried it once (using BG2 classic on Linux), and wasn't able to get multiplayer to work.)

At this point, you're no longer playing the same game.

(Worth noting that I dislike many of the rule changes that were introduced with the Fixpack and with the EE.)

Sure I am. I am playing bg2 with a custom party. Surely you aren't suggesting someone that creates their own party members isn't playing bg2?

If you can't get multiplayer to work in Linux I think that hardly qualifies for making your own party "shenanigans".

I don't play fixpack or EE.

I'm arguing that, once you start modding the game, you're no longer playing the same game.

Enhanced Editions allow you to create a party of any size, even in single player. You do not need to go into the multiplayer menu at all, not even to create a fake multiplayer game that no one else ever joins. The extra characters follow all the usual rules, so it is functionally equivalent to multiplayer, but more convenient. (Among other things, there is no need to keep checking all the "Ready to play" boxes.)

Not only do I find it a terrible argument that you consider a custom party "modding the game", when it has been done from day 1 of the series without any kind of editing of any files, I also find your conclusion that I am no longer playing the same game, to lack any premises to back it up. The difference between a custom ranger/cleric that I make in multiplayer and a modded Jaheira into ranger/cleric is functionally 0, except for banter and quests. Where did this idea come from that playing single-player and taking along npcs in this game was the way the game was "meant to be played"? Please provide sources from developers.

I'm not saying that a custom party is modding the game. What I consider modding the game is when you change the class (or other stats) of a recruitable character, which can't be done without editing a file.

(Also, worth noting that I consider installing a fixpack to be modding the game, even if it may be argued that it makes the game closer to how the developers intended it.)

Some of the recruitable companions have special features that the player cannot replicate, so modifying the game to change those companion's classes produces a distinctly different result than just creating a secondary player-created-character with the target class. For example, Jaheira gets Harper's Call, which is otherwise not obtainable. Minsc's special berserk semantics when his chosen "witch" is hurt are also not directly replicable in a player-created-character.

Harper's Call can be removed, as can Minsc's innate berserk, or it can be added by script to a player character in the case of Minsc, or added by Shadowkeeper in regards to Harper's Call.

I never said it wasn't modding the game to mod npcs into different classes. In fact, I used the word "mod". I'm saying there is 0 difference other than banter and quests between a custom character and a modded npc.

You can of course replicate all features of ingame characters, just not with the ingame character creation.