1. BIP 39 is still the official bitcoin standard.
BIP39 was also made by Trezor developers, and it is one of several BIP standards used for bitcoin seed words.
3. SLIP 39, at present, is a company-specific (Trezor) standard.
SLIP39 is also compatible with other bitcoin wallets so you don't have to worry about that.
Electrum and Blue wallet will work just fine, and I am sure other wallets are going to support it soon.
4. If you lose your Trezor hardware wallet, you will have to buy a new Trezor hardware wallet to rebuild your bitcoin keys from your 20-work seed phrase.
No, not even all Trezor wallets support SLIP39, only newer models.
You can recover to any other supported wallet, and you should find more information on Trezor website:
https://content.trezor.io/slip39