I am planning a trip ahead. I'd like to Enter to Romania via Greece (Single Entry Schengen Visa) Would then my Schengen Visa become Invalid and I won't be able to come back to a Schengen Country?
The Schengen visa would need to be issued for multiple entries as per Romanian regulations https://www.mae.ro/en/node/2040 which say:
Romania acknowledges as being equivalent to its own national visas, for transit and for stays up to 90 days per period of 180 days the following:
- The two or multiple entry short-stay visa issued by a Schengen member state;
Attention! The number of entries as well as the right of stay established as per the Schengen visas must not have been exhausted. The right of stay on the territory of Romania shall not exceed the right of stay granted as per the visas/residence permits issued by the Schengen Member States.
So you either need a two-entry Schengen visa plus a separate visa for Romania, or a multiple-entry Schengen visa. Even entering Romania with a single-entry Schengen visa is not allowed unless you have a separate visa for Romania, as the single entry would have been exhausted when you entered Schengen, of which Romania is not a part.