Point #1, yes, but the difficulty of Bitcoin Cash can only increase after 2016 blocks. So it is not immediate. The difficulty can downgrade in 12 hours, but the difficulty cannot upgrade in less than 2 weeks.

Point #3, Bitcoin SegWit difficulty cannot reduce until the end of 2016 blocks is reached. If substantial mining power leaves, the end of 2016 blocks is greatly delayed. The more it is delayed, the more possibility of miners leaving. It would only take the top four miners leaving in the first few days of a difficulty period to take away 50% of the hashpower, doubling the adjustment period. None of the top four miners presently have any ideological commitment or interest in SegWit.