Albeit the games could be played independently, the "engine and systems" remained virtually unchanged (slightly refined) contrary to your pretence of significant change to lend credence to your sentiment.
That is not correct. While both games run on infinity engine, BG2 one is significantly updated both in terms of visuals and gameplay. This is less pronanunced now, as EE editions use BG2 engine for everything and even before players used Tutu mod to play BG1 content in BG2 engine. Mechanics and classes were far more limited, no dualweilding was supported, no subclasses were present. I am sure there are more things I am missing, but it's been a long time since I played vanilla, unmodded BG1. But there is a reason why I never managed to get BG1 vanilla to run properly on modern systems, while having no issues with BG2. Oh, no highlight feature was available in BG1 (therefore the existance of hidden powerful items in small spaces aroud the maps) though if I remember well, that feature was added to BG2 with ToB expantion.
I'm not a native English-speaker, but my understanding of a self-contained story is something COMPLETE and SEPARATE with its own ENDING. BG1's story was clearly subordinated to the overriding story arc of the series albeit having different stories tied to different villains.
We might argue here about deifinition, but I don't think self contained story means that no threads a left to be continued. BG1 is about conflict with Sarevok and your character's arc is complete after BG1, and BG2 introduces new conflict and villain. Same with Throne of Bhaal. All three are seperate and independent adventures. Throne of Bhaal less so, as it's main focus is tying up the overarching story rather telling a story of its own.
You seem to have forgotten BG2 was comprised of two games; Shadows of Amn, and Throne of Bhaal. Both titles could be played completely and enjoyed as standalone games. The infinity engine and the system changed rather more for the latter than it had between BG1 (Tales of the Sword Coast) and BG2: Shadows of Amn. The story of Throne of Bhaal was as much "self-contained" from Shadows of Amn as that title was from BG1. It is merely the conclusion to the saga, the ongoing story.
Using your selective logic, your criteria for what constitutes a full game/independent title as opposed to an expansion/DLC, then Bioware was wrong in not naming Throne of Bhaal BG3.
Throne of Bhaal should have been BG3 - it was far too rushed to be succesful in properly wrapping up the story. But it was an expansion, it was far smaller in size, adding just a few new systems, and requred BG2 to be played, as it is just new content added to the same game. There was a pretty significant overhaul engine overhaul between BG1 & BG2, even if basics where the same. Same as Diablo1 vs Diablo2. ToB is just additional campaign running the the same engine, systems and tools. It expanded content of base game.
Using your selective logic, your criteria for what constitutes a full game/independent title as opposed to an expansion/DLC, then Bioware was wrong in not naming Throne of Bhaal BG3.
No... Full games are full games, expansions are expansion and DLCs are DLCs. They produced and released Throne of Bhaal as an expantion to BG2, so that's what I call it. They designed BG2 as a full sequel, with introduction to newcomers, and it's own plot, villain and character arc. I am not sure, if narrative continuation has any weight. By your logic, Pillars of Eternity2 is an expansion to Pillars of Eternity2, while White March is a sequel?
