A check valve on the HTL would keep it at whatever position it was in when you disconnected, but you will loose the ability to adjust it. Whenever I am using my 3rd SCV for something other than my HTL, I put the old manual TL back on.
My third SCV control is right next to my rockshaft control. A diverter valve on the joystick would be more convenient, but it would have cost a lot more than the 3rd SCV (at least on my tractor).
I had a grapple from Attachments International that I bought on eBay. It was fine on my Bobcat (before I sold it), but a little heavy for my tractor. I like the looks of this Millonzi.
RE: hydraulic overload. Nothing to worry about. If you pick up more than you are properly ballasted for, your rear tires will come off the ground. Just drop your load and take a smaller bite. If you have more than enough ballast for maximum capacity, your hydraulics have a pressure relief valve that will kick out and prevent the operation of the unit. In this case, the answer is the same, take a smaller bite.