are you saying that this was a partnership that elected S-corp taxation?
if they argue its a partnership/LLC
then 6103(e)(1)(C) would apply
(C)in the case of the return of a partnership, any person who was a member of such partnership during any part of the period covered by the return;
did you read my previous thread?
you can write the CPA you are willing to reimburse for the cost of a copy of the return. (it should be reasonable). use a method where you can prove delivery. if you still get a "NO" or no reply you'll need to see a lawyer and/or file form 4506 with the IRS. the lawyer could advise you about writing a letter of complaint to whatever state agency governs professional registration, a letter to the state's CPA society. even a letter to the IRS about the code violation (don't do this without the lawyer's advice - you may wreck the business and lose whatever you invested)
all this will take time and of course the expense of a lawyer., if you need one. I would say write the letter and file form 4506.
the 4506 can take 75 days
https://www.irs.gov/pub/irs-pdf/f4506.pdf
while you are entitled to a copy of the return, the real issue is the mortgage lender. there's really no way for them to know that what you would give them is actually what was filed or if it is that it wasn't subsequently amended with substantial changes. of course, submitting false documentation for a loan is probably criminal.
