I am only 5'10 and I think its cramped in there !.
That has to be a recapitalized unit as it looks like new. Yup you got the rare hein werner ripper bucket. Its a beefy rig. I saw reference somewhere that they only came with the stock bucket and if anyone in the army tried to order one, they had to have a darn good excuse. The trade off is that it doesn't cut a very neat hole
My local soil is mostly granite boulders and its capabilities get used. There is a technical report in the Misc tech forums that compares the ripper bucket to the regular bucket in digging permafrost in Alaska. The conclusion was the bucket is not the limiting factor, its the amount of downforce you can put on it, when I am really going for it on rock, I end up dragging the entire SEE towards the hole. I needed to dig a trench across a large piece of granite ledge one day, it took awhile but I scaled off about a foot of that ledge.
With respect to the remote throttle, the easiest fix is that the throttle linkage is misaligned. The only way the remote throttle will work is if the hand throttle is about mid travel (well above idle). The linkage is very complex and on occasion things bend and the hand throttle will be off. Given the hours and condition of your rig I doubt its that simple.
The remote throttle will not work if your air pressure is low, as long as its in the normal range and the warning light is not on you should be good. The SEE preferentially saves air pressure to supply the brakes and turns off the air to the remote throttle 4WD and Diff locks if the air pressure drops.
Next easy issue, there was an Army technical tip that the remote throttle switches were failing from water getting in them. The inventory got switched to new supplier and it was recomended to change them out with the new style. I switched mine out with a switch for marine use when it fell apart when I removed it. Those collected tips are in a Misc Tech Section, I got them from someone else on the Benz World site and assembled them in one PDF and indexed them.
The final thing to check is if the remote bucket switch is working. You should be able to lift and drop the bucket remotely from the backhoe seat with this switch. If I remember the remote tool hydraulics solenoid is another way of checking but having gotten a shower of hydraulic fluid from a hose connection I suggest avoiding that test
If the bucket remote doesnt work, then my guess is the fuse that supplies the systems on the rear deck is blown. With the hood off and standing in front of the SEE it is the red 16 amp fuse in left most slot on the right side fuse block. If it's blown you might have another issue with the hydraulic oil fans but I wont go into it until you report back.