Everybody uses the FNMA guidelines even if they intend to hold the mortgage, because if the loan doesn't conform to FNMA then they *can't* ever sell it in the future.
It is futile and senseless to rail against the guidelines -- they are what they are -- just like it's senseless to complain to a waiter that the dish you want isn't on the menu.
Learn what the guide book says and conform to that and you won't have any trouble. We refi'd 3 times after retiring. The 1st time was going nowhere until somebody at the lender talked to the right underwriter and found out what the appropriate rule was. The next 2 times, I knew the magic words to say, and the loan sailed through.
1) You have to have a *documented* periodic monthly distribution from a retirement account (401k, IRA, etc.). For some stupid reason, it has to be a RETIREMENT account; a regular taxable brokerage account won't do. Yeah, stupid reason -- but it's their reason and you have to accept that. They want something that looks like a paycheck -- regular amount, regular schedule.
They treat that as if it was a paycheck -- and they understand monthly paychecks. Sometimes, they want to see that this withdrawal shows up in your checking account. For me, they did once, the other times the broker's confirmation of instruction was all they needed.
So you show them the letter of instruction that you gave to the broker or administrator of the 401K/IRA. In my case, the broker sent me a letter back confirming those instructions and that's what I showed the lender.
2) The account balance must be large enough to sustain 36 monthly payments.
After you get the mortgage, you do *not* have to continue the monthly distribution; you can cancel it immediately. For one mortgage, I still had the first monthly check uncashed, so after the closing I called the broker and told them to reverse the withdrawal, which they did.