RootsMagic itself has neither an online editable database nor is it a collaborative system. Some people have tried to collaborate by sharing a RootsMagic file on Dropbox but it is fraught with risk. There are collaboration systems that could support controlling access with a checkout/in mechanism so that only one person is allowed to modify the shared database file while it is checked out; however, they could be costly.
You can interface a RM database with FamilySearch Family Tree which is a very open collaborative online database that any subscriber can modify. The collaboration would be on FSFT and any subscriber could match people in their RM database with people in FSFT; the RM database would be a very tiny subset of the FSFT one world tree.
Alternatively, you can interface RM with an Ancestry Member Tree. The AMT can be shared with editors, contributors or viewers. Collaboration is again possible online among a restricted few users. However, I am not sure whether an AMT allows or reliably supports TreeShare from multiple RM users.
As to your second question, I don't know.