OK, so I didn't break the game at least. Just finished playing it through and there are no errors related to my meddling. So, here's the deal how to do this:
1. Enter Mass Effect 2\BioGame\DLC folder.
2. Check each DLC subfolder. It should contain its own CookedPC and in some cases, Movies folder.
3. Move all .bik files from these DLCs' Movies folders to Mass Effect 2\BioGame\Movies. If any files repeat, you may overwrite them or skip -- it doesn't make a difference except for the datestamp as any repetitions are identical files.
4. Remove the DLCs' Movies folders so that they don't get detected.
5. Enter Mass Effect 2\BioGame\Movies folder and remove: AdvCombat1.bik, AdvCombat2.bik and AdvCombat3.bik.
Do not remove AdvCombat1_PC.bik, AdvCombat2_PC.bik or AdvCombat3_PC.bik because the game uses these!
Size results forthcoming. Gimme 4 hours
EDIT: Okay, so instead of a 750 MB Setup-3a that I had on the previous run, I have a 410 MB one. I think maybe FreeArc would've done a better job. REV0, any chance for FreeArc version of this installer?
EDIT II:
Also, this goes to Registry for Polish:
Root: HKLM; SubKey: "SOFTWARE\BioWare\Mass Effect 2"; ValueName: Language; ValueData: pl; ValueType: string; Flags: uninsdeletevalue uninsdeletekeyifempty; Languages: pol; Check: Component2FAE;