Material.write_thumbnail is bugged in its design. If I remember it preserves the aspect ratio but exports the largest dimension no larger than 128px (or something, at least the cause why it fails most of the times is when one dimension is rounded to 0). I'm still searching for the detailed forum post and bug report on this. Don't use it, but use Jim's suggestion with a material applied to a group. It will retain the color shift and colorized properties.

I've decided to abandon the tool route. By running outside the tool stack I am able to change the view asynchronously, so the user can navigate without having to explicitly invoke or change tool modes. It's a much more fluid experience, and with the exception of a few minor cases that I think I can work around, it doesn't disrupt tool operation. Thank you for the useful tips, everyone. This is a really fantastic forum!