Hi,
Our current installers require admin rights indeed. That's the default for a Windows install, and what most users have. And I know it sounds strange, but while you are not the first, I think I can count in 2 hands the number of issues with this in more than 20 years developing FlexCel and many tens of thousands of customers. Yes, it is a rare thing 
And at least in FlexCel case, the only thing we use admin rights for is to create symbolic links, and you don't need those rights if you are in "dev mode" (see the first answer here: batch file - Allow mklink for a non-admin user - Stack Overflow ), so that might be another reason we don't see this issue often. If you have dev mode enabled, the setup should run without admin.
But we are indeed aware of users who don't get admin rights, and that was one of the reasons we developed a completely new system, which, among other things, doesn't require admin rights at all (it took a lot of effort and internal discussions to get completely rid of admin requirements) . See TMS Software | Blog
So my advise if you require admin rights, at least for the products that are currently supported (and FlexCel is one of those), just use tms smart setup. While labeled "beta" It is very stable right now, it doesn't require admin rights, it is much faster, and it can install everything in a single command. Myself, I can't see me going back to using the old installers for any product which is already in smart setup. And it is the future for all our installers, setups.exe are going to be slowly deprecated.
There is a link to download it in this post:
Completely uninstall FlexCel (so we don't have repeated things in the disk), then open a console, and in a folder where you have full access as non-admin, type tms install tms.flexcel.vcl and that's it.