Ok, a little more info to work with. At this point I don't know if I'll ultimately be able to help with the scope of the problems you're having. We must consider, being a used machine/attachment, neither you nor I have any clue of actually how many hours the machine has been in use, how it was maintained, the previous machine(s) that worked the unit (pressure, flow, condition of the oil, what size case drain line was used), and how many components are original or have been replaced.
What we can do is work with what is common knowledge. First, case drain line. Gear motors do not require a case drain as they (by design) vent the oil used to lubricate the shaft bearings back to the return oil side of the motor, basically looping the oil. Other motors, piston motors, axial motors, require a case drain. Take a piston motor, the working oil passes through the piston chambers and pistons to do the work, controlled by a valve plate. But the other parts of the motor, such as the shaft bearings and slipper shoes, are not exposed to the oil inside the piston chambers, they would have no lube. So by design these motors are made to have a certain amount of "internal leakage" to allow lubricating oil into the entire housing of the motor to lubricate these parts. That's where the case drain line comes into play. Without a case drain, pressure would build inside the motor housing and blow out the shaft seal.
Knowing this, if you blow out the shaft seal, likely causes would be 1) excessive wear in the working parts of the motor, too much internal leakage, more leakage than the case drain line can handle and thereby building pressure inside the motor. Basically the motor is worn out and needs replacement. 2) Case drain line too small for the amount of oil coming out of the case drain on the motor. You stated you have a 3/8 line for the case drain. In most cases that's plenty enough ID for a case drain line so I don't think that's your problem, but with no real knowledge of that wood processor I can't say for certain, but that's typically plenty big enough. 3) Something obstruction blocking the case drain line not allowing flow. I'd say you've already verified this isn't the case.
So back to your machine and possible causes. Assuming the 3/8 case drain line is plenty big enough, and you tee into the Bobcat drive motor case drain line, I'm thinking that should work. If I'm not mistaken, that Bobcat case drain line is at least 3/8, near certain it ain't 1/4, so it's really not adding any restriction to your additional case drain. Moreover, the Bobcat hydraulic reservoir is vented, not pressurized, so the case drain is pretty much free flowing back to tank. Granted, since it enters the "bottom" of the hyd reservoir, the case drain from the attachment does have to overcome the weight of the oil in the tank, but if you think about it, would Bobcat run their drive motor case drains lines to the bottom of the tank if that were a problem? Not likely.
So, assuming your case drain line is plenty big enough, and assuming it's really not a problem to connect it to the bottom of the tank, after all, Bobcat does, then it's possible we're looking at a worn motor producing too much case drain oil. Again, I'm not familiar with the set up you have, but a typical diagnostic method is to test how much oil is coming out of that case drain line. Disconnect the hose that connects to the case drain port on the motor. Connect a short hose to that case drain port, a hose that you can drop in a 5 gallon bucket. Operate the motor and monitor how much oil comes out of the case drain on the motor. I can't say what "normal" would be for your motor, but just to throw a number out there, from small motor to large, anything from 1 qt a minute to max 1 gallon a minute going into the bucket would be acceptable, and that 1 gallon a minute is a lot. But if you're getting 3, 4, 5 gallon a minute out of that case drain line while the motor is in operation, that motor ain't happy inside.
I hope this gives you a little info to work with that will apply to your machine.