I understand that 7Zip might use problematic unpacking mechanism, but it is widely used software and obviously it's not going to be changed on the 7-zip side (as it works this way for all the years), so I think Multi Commander should handle it somehow, because other file managers are handling it correctly so as a result Multi Commander's behavior is broken. Worst is that user might not know that the operation wasn't really successful - I was so confused when I realized that the files weren't extracted correctly and some files were missing. I wouldn't expect that behavior. And seeing that it's this way for 6 years without any fix or workaround is really bad.
Your recommendation to use built in 7-zip support isn't comfortable at all for me. I usually download some archives via internet browser, then open them in own 7-zip window, navigate in file manager to specific folder, and then drag&drop them there. Having to search for the downloaded archives first takes 2x more time than drag&drop. But as I said, main problem is that it is broken in Multi Commander in a way that it looks like it's working, but in reality you get lost/corrupted files.
Can you please make it work somehow, even though it won't be ideal? For example make all these drag&drop operations blocking - it's much better to wait than to get corrupted/missing files.