Here is what the wiring excerpt for the ignition interlock looks like from the TM1590:
And here is the starting interlock excerpt:
...we will have to look at them together as each shows only part of the picture on why the ignition is killed if you turn on a PTO or take the ground speed out of neutral. Note that in the second diagram there is 12 volts on the key switch terminal S2 (the #710 purple wire) even when the key is in the RUN position. The upper diagram also shows this #710 purple wire, but not why or when it is 'hot' with 12 volts.
In the upper drawing, relay contacts A must be closed to enable the ignition to make spark. IF the transistor C is conducting, as it will be when key switch S2 is hot supplying voltage to TDCM X23 pin 7 via the #705 purple wire, the engine will continue to run -- INDEPENDENT of the seat safety detect enabled at TDCM X22 pin 9. If the seat safety conditions are satisfied, that pin 9 also receives 12 volts, the one-second timeout in the TDCM is inhibited and transistor D also conducts. You must have either transistor C or transistor D conducting to keep the ignition enabled. If there is no 12 volts on connector X22 pin 9, then opening anything in the series circuit in the lower drawing will kill the engine. If your 420 is below S.N. 595881 then you do not have the parking brake switch, but turning on a PTO or moving out of Neutral will definitely remove voltage from key switch terminal S2 and therefore turn off transistor D.
Use a voltmeter to measure what voltage you have at TDCM X22 pin 9 when the key is in the RUN position -- if it is not 12 volts with the seat switch bypassed as you believe it is then there is an open in the wiring on the #800 or #810 pink wires, or a bad fuse at F2 (or an intermittent fuse holder if it is the older plastic tube style...)
Help me here with some feedback of what you actually find on your particular hacked wiring with your observations and voltage measurements and we can continue to troubleshoot if needed. Like Hank says, we can only tell you where to look first -- where to look next is dependent on what you find at each step...