Horizontal Sliding Wood Window

Any hope for this thing?

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building is at least as old as 1881 and was redone in the 1970's. I'll paste the descriptions from that page if you don't want to load the pictures:

"We will do a big remodel in 5 or 10 years so I don't want to replace it now.

The top window pivots from it's midpoint. The bottom window slides.

Here's the top window in an open position. Of course we want it to open the other way so it sheds rain and falling leaves. It seems a shiplap type of trim inside and out will secure all but a few inches at the central pivot point while making this position impossible.

To the left is the neighbor's house and my temporary scaffolding below (20 foot drop). Both windows have sills below with some overhang but only the top one has a significant slope and a curfed notch below the ledge to prevent water flowing back and force it to drip down.

This is the existing top trim piece (nothing fancy here). The side trim is missing. Should I do some kind of flashing or just caulk it? This is the sliding window fully open. The front/left panel can be caulked and painted shut, the back/right panel is very difficult to weatherproof, especially since this faces west where the worst winter storms blast it. The sliding panel slides through the half-round encased slot with loose screws laying in it. Is this hopeless and I should just paint it shut?"