A month later, and I finally got a chance to install Ecosystem and give this fix a try. It worked! The errors seem to be no more.
Incidentally, I mentioned not being able to get the plane to orient itself to the path, despite having "Orient to Path" checked. I found the answer to that problem here:
http://hutonggames.com/playmakerforum/index.php?topic=3359.0...where courtneyrocks says:
You have to include a Look Ahead value. I think this determines how far ahead on the path the object is looking. I ended up using a a value of 0.001 and it finally works as it should.
Vitally important info, IMO. Obviously it's more an issue with itween than with Playmaker, but on Playmaker's side, in the tooltip for the Look Ahead value, which currently says you can enter a value between 0 and 1, it would be really helpful if it mentioned that 0.001 is a good value to for getting the object to orient to a path without any derivation - the way I would think most people would expect it to work.
Normally I can just trial-my-error my way into figuring something like that out, but I tried 1, then 0, then .5 then .3 ... None of them came close to working. With 0 having not worked, it didn't occur to me that a tiny fraction higher (.001) would work perfectly.
Thanks again!