Absolutly no problem!
You Drag & Drop directly your files stp, sat from MOi inside 123D.
(Drag & Drop is just more easy but you can use Menus also)
Modify them.
Export as STP & SAT for MOi from 123D.
Save as .123dx from 123D if you want keep the scene in the owner format.
Exit .
it reload the .123dx saved by Drag & Drop...
Absolutly no problem!

Hello, and Yes! I encountered this problem as well! However, I noticed if I wait enough ( I mean for a longer amount of time) the program will be closed eventually.

Hi,

Block in your firewall 123D Design to connect to internet and you'll notice that it will close as expected.

Cheers