If I can buy dust for BCH and eCash, transfer them away to new wallets inc dust, then would BCH and eCash be safe?
Yes. Any transaction that includes an input that doesn't exist on another chain, can't be replayed elsewhere.
I.e. if I then moved BSV without dust, what would the risk be?
That should be okay if there other Forks have moved already. And that's usually the order in which I move Forkcoins.
I'm looking at Changelly or similar to swap some of the low cap coins. Selling options particularly restricted to me in the UK these days. Will do in small chunks of course.
Bestchange may show more options, but all of them require the same caution.
I did read your send-to-pubkey post, that makes sense. If I have the signed transaction from my offline BCH wallet, can I then broadcast it from
https://blockchair.com/broadcast (selecting BCH network) as I did with the BTC process?
Yes.
The BCH chain is a lot smaller than Bitcoin.