1. if buy a new phone and install trust wallet, memorize the phrases, receive some tethers and then stop using the phone for good until a future time: is there any chance of your trust wallet getting hacked?
No. But, you would more easily forget the phrase as human memory is not reliable as a back up, much less as the only means of access.
The fact that it's sometimes called a mnemonic phrase does not mean you should attempt memorizing it. Always use a form of offline backup(s).
2. do people keep millions worth of crypto in trust wallet?
Crypto are stored on the blockchain, wallets only allow you access it through your private keys.
Preferably bitcoins worth above $1k should be stored in a hardware wallet or airgapped device.
3. any chance of your trust wallet account getting frozen like they might do on exchanges?
On a custodian wallet, the risk for this is very present and high. Trust wallet is non custodian, so not likely. But, it's not recommended cause it's closed source.
