Find another watchmaker would be my advice, because this isn't a difficult thing to solve, and again you don't want top push this guy to do something he's not comfortable with.
But I'm not quite sure what his diagnosis means. If the screw is rusted, he would know that since the symptom would be that it can't be turned. So when he says it's the screw
or the setting lever, it makes me wonder what exactly he has had difficulty with because the screw should be obvious. If he can turn the screw, then it's not the screw or the setting lever that's the problem.
The approach to solving a situation where the stem cannot be removed depends a lot on the construction of the watch. Sometimes (like in the example that I mentioned previously) it's possible toe completely disassemble the watch down to those last few parts while it's still in the case.
You can then address this by soaking in something to loosen the rust, which is what I did. But the Speedmaster is a different animal obviously. You can't remove the dial by removing the bezel and tilting the movement as I did above, so my approach would be to remove the enlargement ring, and use the gap this creates to get in there with a screw slot file, and cut the stem.
Should take maybe 5 minutes to get the movement out...
Cheers, Al
