You have a very complex situation to tackle. When you go to these mortgage people what they care about first is if they can make a nice commission or not. If not, pushing you out the door so they can move on to the next person is what is mostly on their minds.
The fact you found out "a few years ago" that you were sold an illegally created lot has me wondering if you bothered to take action over that or just blew it off. It is going to take money to correct this, who is going to pay for it? Did you bother to see if you had a winning lawsuit against the people that sold you that property and the title company or attorney that did the closing? Did you get an ALTA policy? You intend to try and refinance a known illegal lot while not comprehending hardly any of the issues you face, or so it seems. Personally, I would probably have sued over that, I would be checking to see if I still could, and I would be speaking to the jurisdictional authority (county planning department) to see what their management thought about a lot line adjustment to create two 4.8 acre parcels deemed legal through a variance process if needed. And I'd be suing the prior owner that sold me that place, and the title company, for all the expenses involved over their lack of disclosure to me that I was being sold an illegal lot. But that's just me.
My friend, you need a local expert on these matters, and possibly legal advice, far more than you need a real estate appraiser at this moment.