Hi 2ndlaunch. Nope, never did figure out the headliner removal issue, and haven't poked a hole in it to inspect. It's on my list.
Interesting that your similar-vintage Pacific Trawler was launched without a kingpost for the mast. Methinks they were not exactly cookie-cutter boats, and each one apparently got different details at launch. Mine is equipped with a tabernacle-mounted mast ~12' high, supported by a headstay to the pilothouse roof, and two side-stays to the cabin sides, with a kingpost under the deck to the salon floor. Also included were two 1000-lb. Anderson manual self-tailing winches for lifting, with a 10' boom for dinghy handling. The mast is designed to be easily lowered for moorage inside. Very sailboat-like rigging.
As I upgraded my dinghy (originally a 10' Caribe w/8 HP outboard, probably about 300 lbs) to a 12' aluminum AB w/30 HP outboard), I very carefully calculated the rig loads (including the compression load through the kingpost to the salon floor) to accommodate the 500 lb. AB dinghy. Accordingly, I concluded the kingpost was essential, but did not need upgrading, but decided to upgrade all the standing rigging to 1/4" Amsteel. And, instead of replacing the manual winches, I bought a WinchRite electric winch handle (~$600), which works GREAT!
I'm not familiar with the St. Croix davits, but as my boat runs a bit bow-down because of the hull extension, I suspect that a dinghy handled off the transom instead of from the boat deck might be a very good option. Absent a kingpost, I don't think I'd attempt to handle a dinghy much above 300 lbs. from the boat deck, even with up-rated standing rigging. Compression load on the mast is amazingly high. I don't think adding a kingpost is terribly difficult (mine is simply a 2" square teak post, butted under the boat deck, to the salon floor, at the forward corner of the galley counter), But that's simply an opinion, of course, YMMV.
Happy to chat more if desired, although I'm in Alaska at the moment, with poor email coverage. But I can take pictures, etc if you wish when I return in Sept.