1. Move bitcoins in Electrum to my new Trezor. [1]
[1] Can someone confirm this will move only BTC, and not BCC, since the BCC blockchain has replay protection.
i can confirm it. BCC has replay protection and so far i have not heard any news of someone saying it went wrong.
3. Enter the seed/key of now empty old wallet in Electrum in Electron Cash. [2]
[2] Since the BTC have been moved to a new wallet, entering an old seed in Electron Cash will not put your BTC funds at risk, right?
this is correct.
although i would like to add a couple of things:
try installing Electron cash on a new computer (use a live linux or even a virtual machine) because it uses the same directories and imports all your wallets into its own directory and may mess things up.
also remember that if you wanted to come back to "electrum" with your bitcoins, make sure to make a new wallet with a new seed. don't use the old one anymore.