You took the words from my keyboard - this is exactly what I need.
How are you going to approach the issue? Are there any other solutions available out there for this challenge? I feel that marketplace is almost there, but it does not quite there.
About the definitions:
1 - check
2 - I don't mind doing it manually.
3 - That's the problem with MP right now
4 - That too
5 - That's a lesser issue for now, IMO.
I think that if issues 3 and 4 will be resolved, UC and MP will be so much more solid than it is now... For one seller, UC is very good, but for multiple sellers it just doesn't work.