Images from outside

Yep. Rake line trim and the lack of a drip cap above the head trim.

Not a window issue. Google "Drip cap" or "Z-Bar" flashings.

Thanks for the feedback. It appears the window head trim, siding above, and trim above the siding would have to be pulled to install the drip edge. Or could I pull the head trim and install a drip edge, then replace the head trim?

Typivally you can just pull the head trim. Make sure the drip goes underneath the weather barrier. There could still be a chance it is leaking at the roof.

I got up there today and I secured the drip edge on the roof and caulked around some of the gaps. It was coming up in spots due to nails and screws backing out. I noticed the siding and flashing is in rough shape up there too. I’ll have to wait for more rain (Oregon, so not too long) to see how it goes.

Caulking is temporary at best. I would seal the hell out of it and add new screws as the old ones are in plywood that is rotting/falling apart and then count on some considerable remediation work in the near future.

I did add some new screws. I tried going into the existing holes but they were not biting. The roof is above a bonus room. Am I correct in thinking that I could pull sections of sheet rock in that room to assess the condition of the plywood?

"I tried going into the existing holes but they were not biting"

Because the plywood has begun rotting more than likely. You can see the lower edge4 of your T1-11 siding is rotting from water splashing on it as well. You can pull the drywall and look at the plywood but my money is on rotting plywood.

I’ve looked into it before and i think it’s cedar channel siding. It’s individual planks of wood that interlock. Regardless, it’s in bad shape and I need to repair or replace. I also don’t know if that flashing where the lower roof and siding meet is installed correctly. Lots to be done. Thanks for the feedback.

Is that rake edge over the shingles?

Yes. It’s a “mono slope roof” and it looks to be a common way to install the flashing on the ridge, https://www.jlconline.com/how-to/roofing/mono-slope-roof-flashing_o

Tough to tell what surface that was from the pics. I'd be very interested to see what was doing under that Drip Edge.

The first photo is the ridge cap on the mono slope roof. It’s secured to the sidewall and the roof shingles. I think the previous owner used too short of screws to resecure the ridge cap to the shingle side. They then backed out leaving a small hole through the shingles and sheathing which ultimately made its way to the window (I think). I resecured the ridge cap and so far so good with that during light rain.

I'm betting you have plenty of rot to go along with that leak. Time for some delicate demolition; you've got to have proof of what's happened and build proper details. No caulk.