Right, back.
I followed the manual install instructions to the letter as requested, + your instructions. Ran the diagnostic tool first, it came back with the same results, apparently not recognising an LCPDFR install and saying the scripthook is found but out of fate, which is odd because I've done as instructed.
Tried to launch the game thinking maybe the tool was wrong (can't hurt to try) and the game just pops up with GTA IV stopped working.
Also loaded the new vanilla install I did again this morning, and again, vanilla runs fine.
I used the xlive.dll file (sounded like the more stable one, recommended etc).
I've used the diagnostic again to no avail.
I honestly at this point have no idea what the issue is. I followed your instructions to try and set the .exes as run on administrator but I am the admin and only account on this machine anyway, so UAC is off, not sure if that honestly makes a difference?
Out of ideas, any help's appreciated but if this is just some mysterious fault that is alluding solution at least we tried!
Thanks.