I admire your ingenuity and fabrications skills.
Plse let me point out one hydraulic aspect so you can at least consider it.
Many backhoes have thumbs to act like your grapple.
The thumbs are usually fixed and not hydraulically powered.
There is a good reason for that.
At this point, to better explain myself I need to wander a bit.
If you connect a pressure gauge tee'd into the push line for your grapple, you will read the pressure of the relief valve setting on your tractor. Lets say 2,000 psi to keep things simple.
I again will use a simple number (4 square inches) for the number of square inches on the piston in your grapple hydraulic cylinder. Therefore your push force will be the system pressure of 2,000 psi x the piston area of 4 sq inches which equals 8,000 pounds of push force.
If you disconnect the retract hose from its quick coupling, you now have fluid trapped on one side of your cylinder's piston.
The area of the piston, when you subtract the area of the push rod shaft, will be less than 4 sq inches, more like 2 square inches.
Now this 2 square inches is having to use a much higher hydraulic pressure to keep the piston stationary. You have 8,000 pounds of push..
The piston is not going to move because fluid does not compress.
You have 2 square inches of area
So you have 8,000 pounds of push being resisted by 2 square inches on the retract side of the cylinder's piston. 8,000 divided by 2 square niches means the pressure on the retract side of the cylinder will be 4,000 psi. The tractor's hydraulic pressure has been amplified 2 x
When you use a control valve to extend or retract a cylinder, the relief valve only provides protection when the cylinder is moving and the control lever is not in a neutral position.
Once you release the valve lever the fluid is trapped and can rise to very high, even damaging levels..
Consider the backhoe bucket thumb. You add a hydraulic cylinder to allow you to easily adjust the position of the thumb. As the thumb closes, the bucket is stationary. The fluid in the bucket cylinder has no where to go. As the thumb squeezes against the bucket you can have serious hydraulic amplification taking place. The relief valve cannot protect the fluid in the circuit being amplified.
I am not an expert in either grapples or thumbs so my words are cautionary. Perhaps putting a few hydraulic gauges into the circuits on a temporary basis will let you know if you can be at risk of damage.
This photo is of a hydraulic double acting cylinder for a front mounted plow on a small tractor.
Look closely and notice the snap ring hanging off the push rod and the cylinder end partially out.
The owner was trying to center the plow blade. When it would not move right, he increased to full rpm's and tried again. At that moment the end of the cylinder was forced out.
This next picture is of the damaged groove for the snap ring. The line for the retract side of the cylinder was not properly seated in the quick coupling leaving fluid trapped in the retract end of the cylinder. When the pressure was amplified, the snap ring destroyed its groove and the cylinder end came out.
This is an area where cross over relief valves might have a use. They are usually found on pickup truck plows to relieve pressure if the blade hits something solid.
Dave M7040