Jeannie, that's a super job of troubleshooting you did. Congratulations on your figuring things out and getting everything to work! Yes, the lowering function of the Up/Down switch simply opens the pressure release valve which lets the top come down. I can see that if the electrical device (which DOES look like a solenoid) that opens the valve is not adjusted to it properly, it would not open.
My trailer's manual release valve is a bit different - no lever, I have a "knob" that is turned. I'll have to see if I can open it up kinda like you did to see how it works in my trailer. So far, I've had no trouble raising or lowering the top, so I've never looked into it.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan, LiFePO4 battery 12V DC Electrical System
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew