To be honest, not sure what’s going on. I’ve never had problems with skinning / mirroring… etc. From your steps that you take, it sounds ok. The only thing I can think from your older shots, is your mirror threshold is extremely low at 0.01. Although the preview bones should show up red when the mirror is not detected as valid, I would try and make that bigger and test it out. The default is 0.5, so in your case it is really low.
I can think that with such a low setting, and you still don’t get the red preview bones, your model must be tiny. And that can be another problem. Since scale matters in 3D. When the numbers gets too low, problems can start to occur. It’s possible Max thinks it’s valid, but in fact, it’s not. I don’t know the thresholds on there. I guess just check the scale and if in fact it is really tiny, scale it up to a more normal scale and try again.
So, I did a test with really small scale, where a 0.01 threshold is marked valid, and it works fine. So I really don’t know what’s up in your scene.
I can check it out for you if you don’t mind sending the file, but I understand of course if you don’t want to. But I can’t seem to replicate this problem.