Hi Tod,
Other than your windows and ventilation issues I hope you enjoying your boat.
I have a 1973 42 aft cabin which originally had what I refereed to as FrankenWindows. Those lovely stainless steel welded window frames with a million screws that promote leaking and unbelievably bad engineering that does not promote opening of the windows. I tried everything to make those old ugly things redeem themselves and finally gave up and bought all new window from a company near Los Angeles called Heit. They are an RV, bus and semi truck window company which bought a marine window company some years back.
The results are fantastic. After eight years the windows still open, close and lock with minimal maintenance beyond flushing the tracks occasionally with a hose. They weren't cheap, a few thousand dollars, but the results are what every 70's boat owner dreams of. If you zoom in on the pictures you can tell which is before and after.
Most likely when your windows were installed they used butyl tape instead of caulk to seal between the window frame and boat cabin walls. That often fails and allows water to intrude into your boat and ruin the interior wood and wood core of the cabin walls. All of that is mostly repairable.
You mentioned double pane windows in your questions. A boat neighbor actually went to Home Depot and ordered vinyl replacement windows like people install on their houses and stuck them on his boat. They looked horrendous and had no business being stuck on that poor boat. Just so you know, I'm not rich and not a boat snob but if you're going to modify your boat at least use marine parts. There are already enough vessels out there with rusty drywall screws and romex wiring installed by clueless or lazy fools who are wasting time by trying to save a buck and endangering themselves and their passengers.
As for venting your head (bathroom). I live in a fairly dry climate in Southern California's central coast so opening the head portlight (bathroom window) after showering is enough. There's also a overhead hatch in the forward head on my boat that gives plenty of ventilation. If you're interested in mechanically venting the room an exhaust fan could be installed. Maybe look at engine room vent fans. They're not quiet but they are approved for boat use.
You also mentioned something about a sliding hatch. If it's rotted wood you can build a new fiberglass one fairly inexpensively.
You've got a great boat there and hopefully you'll enjoy it for many years. I'm happy to answer any questions or send more pictures of the work I've done on mine