Hi Pat, venture to say here that the stanchion base bolts are threaded to a metal plate bedded into the deck, like ours. Our model is a bit different than yours, but the stanchion arms on our boat have flat head machined screws that are threaded into a metal plate bedded into the deck. No need here to open up the inside of the boat headliner to gain access to nuts, which do not exist.
Have you tried to remove the bolts, and if so, what did you find? In other words, is there any kind of stanchion base fastener on deck that you can undo either with a wrench or driver? If not, give it a try. What is wrong with the stanchion base in question?
Before you open up the headliner, contact Marlow Hunter Customer Service and ask them for the best practice to remove said stanchion base.