At first glance, your trailer is too short. The bunks need to all the way to the transom. Below is a picture of my 2019 2520XLD with twin Yamaha 200s. Notice that the front axle is just behind the pilot house. 6000 lbs axles also.
Leave them neutral. They aren't needed on a twin engine setup. When mine started to wear out I cut the tabs off flush with the anti-cavitation plate. The important thing is the little scoop on the leading edge. It pulls water up to cool the lower unit. The more parallel to water flow it is...
Was everything and everyone in the same place when you took the two measurements? Did you drink all the beer out of the transom cooler, drain the waste tank or the water tank? The boats attitude in the water will affect the gauge readings.
You cannot make any readings through the fill port...
Later on I wound up changing the wiring a little bit. I added a FLIR camera and didn't want to cut in a new switch. I used the STBD wiper switch for the FLIR, relabeled the switch and tapped into my electronics switched bus bar to power the wiper controller.