I appreciate your comments & taking the time to do a detailed response. The fact that it didn't talk down to me as I've had in the past is also appreciated. I'm here to learn, not be right! I learned about MWBCs in this thread and have taken steps to remedy any mistakes. For this a appeciate the time & efforts of all those who posted. The last thing I want to do is fry the family.
I'm even going farther than the electrician suggested & double checking that whatever circuits I have on the Left lug are on the A gentran lug and those on the Right lug are on the B side. This is to ensure that any circuits routed through the gentran panel maintain MWBC polarity for when I do use the larger EU6500. There is one minor lighting circuit that on second look may not work out due to an insufficient number of B side breakers, so I may remove it. I called the electrician back on this who sees no real world issues, but I like to be safe.
Your analysis matches mine. I do see where the other pros here are coming from & agree that there would be a danger with a larger generator. There is NO WAY I'd put 40 amps through a 3 to 4 wire adapter set up. As a result of this thread I've cancelled plans to purchase a second EU2000, which I had toyed with using in dual synch mode for 27.4A (3200W) continuous / 33.2A (4000W) surge output. That's too much, which I won't risk.
Glad to see your EU2000 worked out so well. I've only needed mine once "for real" and noted a drastic cut in fuel use over the already very efficient for its size EU6500 I also own. You used yours for 12 days. At 18 hours use per day that's about 2 gallons. To do the same with the 6500, never mind the gas guzzling "standard" gen sets and we're talking 6 gallons, a 3 fold increase. A cheaper "standard" gen set would use perhaps 12 gallons per day or more! Over 12 days thats a lot. Again, it isn't the fuel use & cost that concerns me, but availability. In most outages gas stations also shut down or, if the outage lasts long enough and as we've seen on the news, run out of gas. The only times I might need the bigger generator is when running the well, microwave and/or heat. But, I have a fireplace, which generally heats the house down to about 35. Below that the heat will tend to come on.
Got to be a fan of these inverter Hondas. Way quieter and more efficient than anything else out there, although I hear the Yamahas are good too. Just harder to get parts for them, which is why I went with Honda.