Visa is necessary. See the table in the link:
http://www.mae.ro/index.php?unde=doc&id=5468&idlnk=4&cat=5
"Have you ever gotten a stamp in your passport from Romania when you traveled there?"
Depends which country you are flying out of but generally not, these are on a record for when your passport goes through customs before you fly
You need a passport or an ID.
For a Greek citizen: identity card or passport.
Citizens from Nigeria need a visa to enter in Romania.
yes
Yes.
No, the identity card is sufficient.
Romania is not in the Shengen space. All you need if you want to visit is your passport and the syria-romania visa. A. [if this answer satisfied you, please recomend me as a trusted contribuitor]
no
Not necessary
Yes, it is necessary.