1) If you forget the pin, you can still recover the account with all the funds in them with the recovery seed right. But if you forget the password then you cannot, right. Is this correct?
Correct, the pin is just for accessing the device,
the passphrase however is a BIP39 passphrase that changes the derived seed from your seed phrase mnemonic.
In other words: Different passphrase will create an entirely different wallet with different set of prv keys and addresses.
So better keep a backup of that passphrase in a separate safe/location.
2) I click on my account on the web wallet, I see a receiving segwit adres. Can I keep on re-using this adres as in sending 10+ in a month transactions to it and everything will be fine as in receiving the funds on my trezor wallet?
Yes, but that's not recommended for your privacy.
3) how can I view private keys of the wallets that are in the trezor and how can I add a bitcoins adres private key myself into trezor?
AFAIK, you cannot import private keys to Trezor.
AFAIK, the only way to view your private key(s) is through the seed phrase (
+ passphrase if enabled) and use a third-party tool that supports BIP39 like "
iancoleman's BIP39 tool";
that will defeat the purpose of using a hardware wallet though.