Yep, I'm the dev.
The issue for installing against native MAME has been reported, and, no, it shouldn't keep counting up with the percentage. If you kill the app via task manager, the files should actually be installed, but the tag in the ini file won't be updated (that tells the app MAME is installed).
I am currently working with two different users on what is going on. I can't replicate the issue here on any machine I have. I have done a ton of tests, but nothing that points out what the issue is.