Honestly, I'd have to look. I have a D4, but I don't remember for sure. Is that display of the input level dependent on the sensitivity settings? I thought it was, so if it's turned way down...
You know that these things are basically just audio inputs, right? You should be able to plug about anything that makes sound into it and get something happening. For testing purposes, you might plug in a phone playing your favorite song or a synth holding a note or something more steady (and hands free) than the cymbal pad so that you don't have to keep hitting the thing as you tweak parameters.
If it really just sat there plugged in, it would be really weird if the jacks broke unless it got bounced around, or the cable was pulled or something else relatively catastrophic happened. It might be that they built up some layer of oxidation on the contacts that's insulating them from the cable. This will often show as an intermittent connection that comes and goes, fades in and out, or crackles and is noisy (if you could actually hear it). They are sort of self cleaning, though. Plugging in and out a few times can scrape away that oxidation and get it to work. Add a little contact cleaner to the plug for bonus points! Just be careful not to put too much stress on it in the process and end up actually breaking the thing!
As far as how hard it is to take apart... It's like anything else. Take out any screws that look like they're holding the chassis together. Then you can pop the top and see what else you need to remove in order to get the PCB out. Maybe snap some pictures so you can see where and how to reconnect anything that you have to unhook.
I would stop and try some things before actually removing the PCB though. It's only 9V and like 1A, so you can mess around inside while powered up without killing yourself, but do remember that some of the components are going to be static sensitive, so ground yourself like you would if you were working inside a computer. You don't have any reason to touch any of the chips anyway... Plug a cable into one of the suspect jacks and then use your meter to test resistance between the parts of the plug on the other end and the corresponding lugs on the jack inside the box. I'd imagine you can do that part without taking the whole thing apart. If it looks right, then you would see if the jack itself is actually connected to the PCB by choosing an appropriate spot on the board to test.
You could post some picks of the guts. If they're well lit and in focus somebody might be able to spot a fault, but these things can be tough to diagnose without actually having it in your lap.