Well, again, I'm not 100% sure how that load sensing circuit works, been years since I've worked on those things. However, we know the dump valve is working correctly. Second, that 2800 psi, that's either we're seeing the sequence valve not working correctly and cramming oil past the main relief valve (just below the sequence valve in the diagram), or, the LS circuit input on the spring side of the, along with the force of the spring, is more than enough to counteract the force of the pump input to the sequence valve and cause that 2800 psi, I'm just not sure about that.
What puzzles me, since you get 2800 psi when you power up the dump valve (close return to tank for P1 circuit) then why don't the bang functions work? I mean, if you select, say, platform tilt up or down from ground controls, if you've verified the coil for platform up/dwn is getting power, have verified the flow control valve is getting power, and verified the dump valve is getting power, then why isn't the platform tilt up/dwn working because we know we are getting pressure when that dump valve activates? If you've verified all this, it keeps bringing me back to the flow control valve is not working correctly, that's the only thing between P1 pump circuit and the platform tilt valve if we have 2800 psi on G2 test port.
One thing of note, the swing and upper lift share the same sequence valve, and the bang functions have a sequence valve, those two valves are the same, you could always swap them and see if you get a different result. Likewise with the flow control valve, there are three, one for main upper boom, one for swing, and one for bang functions, same valve, you could swap them as well. If you do decide to pull those cartridges be sure and look at the o-rings and back up rings closely looking for signs of failure.