I might be totally off, but a few advices for you :
Make sure that the command in the .bat file do NOT have the /s parameter - as in your sample.
/s means that NO message will be shown to you and you won't recognize a failure.
On Win7, regsrv32 behaves a bit different.
a) You must specifiy the FULL path to the file
b) You must rightclick on the RegIndeo.bat file and select 'Run as Administrator'
So, something like this should do it:
From SystemShock2 I learned that the 'Thief'-engine ALSO may require to be run in 'Admin' privileges.
After you have installed a FM, rightclick on the Thief2.exe (or t2x.exe ??) and select 'Run as Administrator'.
If this fixes your problem, then you might want to do this for a permanent change:
Right-click on the Thief2.exe/t2x.exe , select 'Properties' and then 'Compatibility' and check the 'Run as Administrator' option.
Here is my Darkinst.cfg. I pointed out one entry which seems to be missing in yours.
I am not sure if the double \\ should be removed or not (like in 'D:\Program Files\Games\Thief\TMA\\t2x').
And while you are at it, you could also replace all 'D:\Program Files\Games\Thief\TMA' with '.'
Maybe T2X has a problem blanks in the pathnames. Don't know.
So, try this: