OK, I'll bite and summarize it all again.
Can I, as the husband of a US Citizen, choose to file as a
non-resident if I filed a "married filing separately" tax return?
You can file either as a non-resident MFS (married filing separately) OR resident MFJ (married filing jointly) with your wife. You cannot file resident MFS because, as you correctly stated, your presence on F1 is excluded from the 183 days rule for up to 5 years.
If you file as non-resident MFS your wife must file as resident MFS (or HOH - head of houshold - if you guys have dependents).
You can only file one return. If you file MFS - you can only file 1040NR (your wife files separately a 1040). If you file MFJ - it includes both you and your wife, and is a regular 1040 (one filing for both of you together).
If I am allowed to file as a non-resident, do I have to pay FICA on my
As we've discussed before, if you're resident for tax purposes - you have to pay FICA. If you're non-resident for tax purposes, you're exempt from FICA because you're F1 (not any non-resident may be exempt).
From your comment:
Re. your questions, all but one of the advisers I met told me I can
only file resident MFS as I was married to a citizen and cited a 183
day rule for the SPT (but I am sure F1's are exempt from that rule for
upto 5 years)! The one that did know that I could file nonresident MFS
suggested I file joint because I would be making SSA contributions
that would be beneficial to my future (I'm not really interested in
that as I still have many years of work ahead of me).
They're wrong, all but one. The one that's right is also right about the SSA entitlements, but that's up to you.
Disclaimer, to be on the safe side: this is in no way a tax advice and you cannot rely on whatever I wrote, here or elsewhere, as it was not written or intended to be used, and cannot be used, for the purpose of avoiding any tax related penalties that may be imposed on you or any other person under the Internal Revenue Code.