Ah, that's one critical piece of information you didn't provide in your post.  Apparently, the employer and location of service had always been in MD but since the reciprocal agreement cedes to the taxing right to the resident state, the result would be just the same as outlined above, and as you said, MD refund and PA balance due.

In terms of withholding, the question is whether your client submitted a REV-419 to effect the change in withholding.  I'm sure you would have asked your client to submit one by now regardless.

The allocation would then be relatively straight forward, give or take some minor adjustment between MD and PA taxable wages.

Sorry I can't help you with the allocation worksheet in PS since that's not the tax produce I use.