The thing I find with reported problems on the forum is that we normally have very little to go by. But we can often reach pretty far with a little deductive thinking. For instance, an ETEC never needs throttle to warm start or to going from forward to reverse and back. So in this case it becomes obvious this 600HOE is not even close to working right. I would add the odd possibility with the clutch, but there is no mention of a problem with cold start.
The next clue is a change a spark plugs having no effect. These are pretty good clues that the Engine Management really needs attention. Unless the owner is into troubleshooting, which this thread has shown no input from the Original Poster in 11 days, I think a trip to local dealer would be a good thing.
The very first thing I would do before even plugging my CanDoo would be to check the idle rpm. If that is not right then there would be no sense for me to look further. Then I would measure the 55Vdc System Voltage. If that was good then I go on to resetting the TPS. If that didn't help, then I would monitor the 55Vdc System Voltage while it was showing the problem, but I doubt that I would even get that far. The majority of problems are easy to find if you know where to look.
Some people like to troubleshoot, and some people don't. The way I see it with no response from the Original Poster in 11 days, my guess is a trip to the local dealer would be a good thing. At least in my neck of woods the BRP dealer is quick to diagnose and fix ETEC problems. In 2019 it has come to the point that the dealers are becoming proficient in troubleshooting the ETEC while quickly falling behind with the older tech like carbs and SDI. It's a good thing in my opinion.