1. If I happen to use any of the generated addresses from BlueWallet that are attached to my phrase, can I spend them using my trust wallet? Since I can still spend anything from my old address using a blue wallet or even my electrum, trust wallet allows me to do the same thing, as I can't fine any of such options on trust wallet.
Trustwallet will only give you knew address. It means that if you fund another address belonging to the same sed phrase, tí will not be able to spend it on Trustwallet.
2. Among the new addresses that were generated, if I happen to receive new coins into them and I want to spend just the coin I received with the new address, leaving the old one that came with it when I imported the address, will it be possible for me to specify which address I want to be debited from? And if yes, will my transactions also be linked to my old bitcoin address, which I have been using in the past?
All you have to do is to use coin control. Wallets like Electrum, Bluewallet and Sparrow have coin control.
On Bluewallet. Click on send -> the three dots (...) at the upper right corner -> select coin control and you will see your UTXOs and manually select the ones that you want to spend from.
On Electrum. Click on view and check ✔️ coins to enable coin control. Click on the coin tab and you will see the UTXOs which you can decide to freeze or spend. You can also decide to freeze the addresses of your choice.
If it is mobile Electrum, click on your wallet name -> addresses. Click on the address that you want to freeze and you will see freeze address below, click on it to freeze the address.
Firstly those address were generated because BlueWallet is an HD wallet and HD wallets always have 20 extra free addresses to use for a new transactions.
HD wallets will generate many addresses. More than millions of addresses which no one can finish using in real life.
As Zaguru12 advised, create new seed phrase on an open source wallet and transfer all your coins from Trustwallet into that new created wallet on an open source wallet like Electrum.