Wow! Thanks for all the info everyone.
Yeah, this house is one giant air-leak! And, yes....there is black mold on the windows & shower curtain in the bathroom. Bad news all around.
If you dug down 3 feet in my yard, you'd strike water. Part of my yard is a small swamp, which excess drains through a large pipe next to my barn, then exits across the street. My backyard goes straight uphill (rock ledge) & then levels out as you get close to my house. Around the house is level, then across the street, it continues downward. With all that rock, my yard is the perfect flood zone. The neighbors say it was nearly a small pond before they filled half of it in. Waterproofing is impossible without a Rockefeller-sized bankroll, as Lsfarm said. (And if I had that bankroll, I'd have a private 300 acre spread somewhere far, FAR from here with a spankin' new mansion right in the center of it!!)
One half of the house is over 150 years old, with trees for floor joists, & from what I can see through the hacked up ceiling upstairs, is post/beam construction. The other half of the house looks like 2 separate additions, done by the previous owner who was NOT a carpenter. I think half my problem is the fact that I have no vent in the bathroom (this is where all the rotted plywood was--directly above the bathroom), & the other half is that there is NO DRYWALL in my bedroom -- just 1/4" paneling nailed directly to the rafters with fiberglass insulation mashed in between. Huge gaps are everywhere. I can peek right up into the ceiling & see the wet rafters. After reading Lsfarm's post about the laundry chute, I now realize I have to close ALL those gaps up there. I've been planning on gutting that whole upstairs forever, but I have to make it a priority now. The staircase leading to the second floor IS my indoor chimney! It's all going straight through the ceiling!
I know the old section has a huge ridge beam, but the other newer section has a massive beam there too, from what we could see after ripping the roof. There was too much wood there to saw through. Either way, ridge vents are out of the question.
My bedroom was used by the previous owner as an "attic" (the entire upstairs [2nd floor] is the attic), so he never used drywall. The rest of the upstairs is a textured cardboard ceiling, half of which fell down from the humidity. That along with other gaps in that area are letting the heated air go right up under the shingles. You can see a large area where the 2 roof sections meet, where all the heat escapes when it snows. It all melts there almost immediately. The old roof already had ice/water shield in place, so I know ice dams aren't causing the leaks.
I was tossing around the idea of the powered fan with humidity control, but thought that it would just make the place even harder to heat. What I really need to do is gut the whole place & start over, but time & money are both scarce. I think this spring I'll throw in the gable vents & probably try out the fan. Sounds like my only quick-fix for the short term.
Thanks to everyone again for the suggestions! Keep 'em comin'!