TheNona
1-which is a priority: solving OG questions or practice tests? I have been studying for almost 2 months and still have six weeks left
This is a false dichotomy. It's not one or the other; it's both! Practice tests should grow in importance as one gets closer to the test date. But the majority of one's prep should be devoted to guidebooks and question practice.
TheNona
2-Is there a difference in practice questions between OG 13th , 12th , and 11th editions? Is it recommended to try questions of the older versions?
There is indeed a non-negligible amount of overlap among these three books. The best option is the OG 13, along with the OG Verbal and OG Quant. The OG 10 used to be frequently recommended, but some of its questions might appear on the GMATPrep.
TheNona
3- Some sites and forums share very old GMAT paper exams dating from the 1990s !!!! Are these still valid for verbal, Quant, and AWA?
That's probably not the best investment of your time. If you want extra question practice, look into
GMATClub tests, Jeff Sackmann, or the GMATClub verbal and math forums.