These mapping issues are not due to the plugin being V4 -- they are simply bugs in V5. There has (still) been a lot of stuff going on in the viewport area, what with the new Neon display mode plugin, and I still find various issues coming and going from build to build. Just firing up V5 to check this again, it seems like everything but cylindrical and custom object are working fine. For a little twist, though, if I enable Neon, then those two begin working as well. Playing around, I'm able to get the rendered viewport unsynchronized with Neon, similar to how it is failing with the plugin's export. So clearly, data is getting jumbled internally.
On the other questions, I cannot make reliable predictions, and so cannot give intelligent answers. McNeel's stated goal has always been that V4 plugins will not be broken by V5, and for the most part, they have kept with that throughout the entire V5 development process.
Next Limit Team