1.
(7-Zip 9.x compatible methods)
yes 7-zip files with a 7z header Hex (37 7A BC AF or 7zјЇ) while MS packs their updates in their own cab format (4D 53 43 46 or MSCF) the fact that the 7zip file manager or 7z.dll supports MS-CAB unpacking (which is written in the 7-zip help file) doesn't mean the 7-zip plugin does, cabs are supported in total7z as it uses Igor's 7z.dll, tc supports cab unpacking via CABRK.DLL or you can use multiarc +the mscab.exe for full cab support.
2. tc80x32beta15.exe is a zip sfx and is unpacked internaly by tc without the need for the 7zip.wcx
3. The 7zip plugin doesn't open cab files tc does it via CABRK.DLL or multiarc.wcx+mscab.exe or Total7zip.wcx+7z.dll (the one from Igor's File Manager)
TC without any wcx plugin can unpack: zip (zip sfx), arj (arj sfx), rar ( rar sfx), cab (cab sfx), ace (ace sfx) tar and gzip files it does it with the help of external dll's:
like CABRK.DLL, TCUNZLIB.DLL, UNACEV2.DLL, UNRAR.DLL
From your comments you probably don't have multiarc or total7z and the problem might be with CABRK.DLL, post your wincmd.ini file and the sfx archive that's causing problems mr. Ghisler might take a look at it but do it in a new thread as it's not a 7zip.wcx problem
from tc's help file
Total Commander has built-in unpackers for ZIP, ARJ, LZH, .RAR, .TAR, .GZ, .CAB and .ACE files.
NDP40-KB2473228-x86.exe is a cab file (.net 4.0 update) you can see the cab header MSCF at offset A400, some time ago MS introduced Binary Delta Compression (BDC) Technology for use in their updates, delta cabs can't be unpacked with any regular cab unpacker that's why you're getting errors with that file.
For more info read:
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=1562