what theory do you think is most likely.

here they are

1.He could have fallen prior to the war as you said and upon entering the crystal cave as one of the master suggested.

2.He could have fallen prior to the war at the star forge ruins. And go to the ruins there.

3.He actually fell on Malachor but was able to keep the state of mind needed for the task at hand(so he resisted the darkside enough not to be fully corrupted and be its slave) It would make sense the he was scouting for a bases prior to officially entering the war and it was here he learned of the true sith and realize he could use the star forge after the war to take over the republic.And it was here he learned of the star maps.

4. Is the same as above but learned of the true sith and the star forge during the war and was able to avoid detection and he and malak was able to land on dantooine.