I just looked at the NY Regs, Man they have a book they call a pamphlet, every area has different Regs
https://dec.ny.gov/things-to-do/freshwater-fishing/regulations
Go to any auto parts store and give them the year and GM 5.0 and they should be able to find Brass or Stainless Steal plugs
Should also be able to find them on line at a place like Summitt Racing or Rockauto
If key switch doesn't work it can be
Ignition fuse blew
Key switch went bad
Neutral safety switch is bad or throttle handle is not in neutral
Connection broke between key switch to neutral switch to Assist relay Yel/Red wire
Assist relay is bad
Battery cable has a problem/bad connection
Battery...
Probably came with AC-932 but later Merc changed them to AC 41-993 and these are Iridium and gaped at 0.060
Finding the NGK TR55IX is used in the Ford F150 1997-2010, as well as older GM and a few LS1/LS6
Guess that said, the plug should work fine
Don't see the weight being your current issue, just wonder why a Verado was installed other than the XS. Just looked up and the Pro XS came into production in August 2007, so may not have been an option at the time
More weight might cause the bow to ride higher, so not your issue right now...
All good, starter has a solenoid
The Assist relay sends power on the Yellow/Red wire down to the starter solenoid.
So jumping from the Red to Yel/Red on the starter solenoid will cause the motor to crank unless the starter or Battery has a problem. Can even remove the Yel/Red from starter...
Pro XS 115 weight 163kg (359lb)
Verado 150 weight 231kg (509lb)
Pro XS 150 weight 207kg (456lb)
The vids on line do not have anyone on the bow, your vid does have someone there
The Lowe boat uses the Pro XS so your a bit heavier. Wonder how/why someone put a Verado on the boat instead of XS...