Ok I can answer all of tehse since I or someone I know has done them

You cant change realms unless you leave your guild
when you leave a guild you lose all unspent contributions
if you rejoin same guild the lost contributions DO NOT come back
If you change guilds or change realms and then join a guild, you carry over all tech levels even if the new guild is not researched as high as your old guild

That should answer every situation you can come up with.