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?
Although the Trust wallet is acquired by Binance which is one of the most reputable CEX they claimed the Trust wallet to be open source and no source code was revealed. Therefore, there's a chance of a possible hack or backlog issues.
2. do people keep millions worth of crypto in trust wallet?
I cant speak for everybody else but according to stats, the number of hardware wallet users increased this year which automatically mean crypto investors are keeping million in hardware wallet these days.
3. any chance of your trust wallet account getting frozen like they might do on exchanges?
There's no chance for that since the Trust wallet is non-custodial.
4. I found paywithmoon and paywide as 2 options that you can fund with crypto to make online purchase? are there any other options? in particular where you can make purchases in store using a crypto credit card?
Why don't you use Purse.io instead?