Your description of "tie ins bypassing the safety valve" and "rearranging them to come off the lines feeding the rear remotes" leaves me uncertain what you have done so far. What do you mean by a "remote lever?" Do you have an available remote on the rear of your tractor? If so, you need a valve (either already there or else added to your tractor) to control the flow from the remote to the third function device. [Two varieties described below.] It halfway sounds from your description like you may not have a remote and are trying "make one" by tapping in to the plumbing (??) Not a good idea. Yet you mention a "remote lever." ??
I strongly recommend that you add a remote control valve to the tractor if you do not already have one and go from there. The MF tractors have a relief valve in the host body of your remotes. With you having already blown seals on a third function device, I have to think you have tried to homebrew your way without using a normal remote or may have tapped into the high pressure line FEEDING the remotes rather than using the remotes as they were intended to be used. There is no such thing as a "remote lever" per se. There ARE normally levers controlling each remote control valve that control flow FROM the remotes to accessory devices like your grapple. The actual remote control valves are right there in the "remote" host body and the controls for those valves are described below. And of course whatever is connected to a remote that does not work unless you use the associated lever... sorry your description is just puzzling.
All that said, the snazzy/classy/elegant way things are done nowadays is to have the control for the third function be a push button on your FEL joystick. That essentially controls a solenoid that operates the actual remote control valve (RCV) that enables the flow to the third function device. The old school way to do it was to have a cable (Massey had them under some sheet metal to the right of the seat usually with levers sticking upward) to control the valve rather than a solenoid, and in either case the hoses ran from the RCV in the rear to the FEL front frame crossmember where quick disconnects were mounted, etc.
Hope this helps. You may want to elaborate and tell us more of what you already did, whether you have remotes on the tractor, etc. in order for folks on here to help you. Your photos always help.
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My photo below shows how mine are hooked up on a MF2660. Should be similar or the same on yours. In my photo you can see the right RCV (remote control valve) is occupied with resident connectors and hoses. The left of the two RCVs is unused at the moment of this picture and is just blocked off with rubber plugs. Those marked RCV2 happen to be going to/from my grapple or 4-in-1 bucket, etc. (e.g. third functions.) The top connector is normally the outgoing pressure flow and the lower one is the return line. In a configuration like mine, there may or may not be another relief valve up at the loader valve fed by the "power beyond" line. You don't need any other relief valve if you connect your third function to one of the RCVs.