While I won't put it past matt Weiner to use *some* misdirection, the idea that Meghan is going to be killed by the Manson family just doesn't work.
1. How would she end up with the Cielo Drive people in the first place?
2. Putting aside the fact that ending the WHOLE SERIES with a violent murder would be disastrous in terms of drama, how would they do that? Would they CGI a grieving Don at the trial?
3. Suppose they come up with a similar cult murder - that would be even stupider.
I do think there's a possibility that Meghan will be killed or even kill herself, but the Manson theory is a non starter.