Gentleman, I would like to make this a three musketeers approach! Also I am anxiously awaiting your findings!
I have a 2013 600 Switchback ES, pro ride chassis, with the same wiring and components as PROTURBO from reading this post at length and reviewing diagrams.
BACKGROUND: I put on 170 miles early this season and in the course of those miles burnt up 4 headlight bulbs, both high and low filaments. From previous happenings on older sleds, I replaced the voltage regulator. After installing, I started up sled, let warm up, hit the reverse button and noticed the gauge died and 3-4 seconds later the engine died and would not start. After much screwing around/reading, I found that if I applied 12v to the fuel pump prime plug, the sled would start and idle but the gauge would not light up at all. While troubleshooting, on a battery powered fuel pump, my ECU let out a molten plastic volcano, complete with magic smoke, yet still ran!
TO DATE (aka since plastic volcano):
3 New voltage regulators
2 New capacitors
1 New stator
2 New relays (located near primary clutch)
1 Used ECU
(all components have tested out good, old and new!)
Yet I still have the same no start, no run condition unless I power the fuel pump on 12v battery.
I am at the point of getting a 2013 harness as well but after reading your issues and reading the post here, https://www.snowandmud.com/threads/battery-light-flashing-and-engine-light-on-2013-pro-800.103115/
Have you tried or measured fuel pump draw? Mine tonight was 4.76amps DC.