I have the 64-bit version of Win 7 Home Premium SP1 and a self-compiled version of MAME64.exe being used with IV-Play.
I did try deleting the .dat and .cfg files, expecting it to ask for which MAME executable to pair with, but instead it just dove right in with extracting the .xml file and throwing the error.
As for the other thing, I have a self-compiled version of mameui64, that I compiled using this list of make arguments:
make -j5 SUBTARGET=arcade OSD=winui PTR64=1 STRIP_SYMBOLS=1
and this time around, it output the "mamearcadeui64.exe" file. It was still named "mameui64.exe" when I did it with 162, so I'm just guessing that something changed with 163.
Anyway, I don't have it running in the background, verified by checking Task Manager. Should I try recompiling it without the SUBTARGET=arcade pair?