Oooh I like the early dementia explanation.
Magneto helping Xavier build Cerebro is one of the least problematic inconsistencies in the series for me. We know in the original timeline, Magneto was imprisoned after the Kennedy assassination. This took place from the 60s to 70s. The next we see Magneto chronologically is in the 80s, when he and Xavier are recruiting a young Jean Grey to the school.
At the same time, Xavier was in a depressive, drug dependent state in the early 70s, before eventually getting out of his funk to restart the school in the 80s.
It is very possible that Magneto was released from prison (maybe it was proven that he did not in fact kill Kennedy), reunited with Charles, and the two restarted the school together, and this would involve rebuilding/upgrading Cerebro. Then the two would have another falling out, leading to Magneto's departure and creation of the Brotherhood of Mutants circa the 2000s.
Bringing in multiple timelines is unnecessary.