MAF question?????????????
from what you've said what i would do is re-run new wires from the relays to the plugs or from where ever those plugs are supposed to get the current from.
anytime, tbh i wouldn't know what to do if i was in your situation, i don't even know how i came up with all this.

keep at it.
running a toggle switch might not really be a good idea as the fans will only turn in one speed setting resulting in over heating in the future and won't even help with the codes.
here's a small idea... get a wire from the battery and feed that circuit.. see what happens.
i would check and change "ALL" my fuses if i were you, there's been situations were ppl had fuses that looked fine but didn't work.
P.S. Try my small idea & let us know.
First I already told you that the P0507 is a thing that will be fixed with the tune ... Its plain english, the meaning of that code is "IDLE Rpm higher than expected" which means the idle rpm is high ... Proceed to another issue, the P0507 will go off with the tune just make sure you mention it... About stalls I say 3600 I've seen that number more than I ever saw 4k though
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