Ok, re question 1.. It depends on the options presented by the DAW when you export stems. Usually the audio stem for each track will be what you hear is what you get, but there may be options like include or exclude plug-ins and volume/pan settings. And if any Group level tweaks are part of any Automation data, there might also be an option to include or exclude that as well.
As far as re-importing them.. Well, if your collaborator didn't change anything in the stems you gave him, then there's no need to re-import them, just import any new tracks he added to the project back into the same session you exported from -- the original tracks (including any Neutron plug-ins and the Visual Mixer) should still be there. However, if you did import the stems back into a new session, they should sound the same but all the effects (and Visual Mixer settings) would be permanently baked into the audio files themselves.
Again, think of the Visual Mixer as simply a remote control interface for the individual output level/pan controls of the iZotope plug-ins in the individual channels. If you re-import the stems back into a different session, there'd be no iZotope plug-ins so no Visual Mixer, and if you did set it all up again you'd be starting from scratch -- BUT the audio files themselves would still reflect the levels you'd dialed up with either/both the channel strip faders and the Visual Mixer when they were exported.
So, exporting audio stems normally renders all the effects and level settings into the new exported audio file -- again, unless the DAW offers options not to do so and you use those options. What you decide to do would depend on what you and the other person need for the collaboration. Of course, if you both can agree to work in the same DAW, then there'd be no need to render/export audio stems, you'd just exchange session files (folders), making life much easier.. ;-)