you may not get OPT after file

You won't be denied because you filed your 485 as F1. That part of what your lawyer said is wrong.

However, it is debatabel whether or not you can keep your F1 with a pending 485. And the problem is, even if you can keep your F1 with 485 pending, you may not be able to extend that status, which means when your F1 expires, that is it.

Given that you are graduating in 6 months, you probably should change to H4. That way, if 140 is denied, you won't be out of status. However, if your main concern is working, then either status won't protect you. Because once you use 485 based EAD to work, you will loose the H4 anyway.