When installing, about 8 out of 21/22 files MMM installs, Windows says are not valid installers (so it doesn't install them). Is this fixable, or will this be patched?
I had no trouble when I did it a while back and I have not had any problems with the installers. However, this week I downloaded the add-on to Movie Edit Pro17 after realizing that I had never done so (only looked at the program back then, didn't use it). I wanted to get any effects that disappeared from more recent versions. 3 of the downloads did not download completely or failed to install. I ran them again and 2 of them worked, 1 to go.
Did you try downloading again the ones that did not install?
If this continues, I suggest that you advise Magix support.
I would ask the question - why get it from Steam other than it is slightly cheaper then direct from Magix - any problems you have downloading are the responsibility of Steam and you should contact them.