He's deeply insecure = correct (He doesn't feel good about himself) He [really] can't be trusted (He's not an honest person)

'He's deeply cannot be trusted.' is grammatically incorrect. In fact, it contains many grammatical errors. It's simpler just to say, 'He cannot be trusted.' A person is generally either trustworthy or not.

No. The first one has strange grammar, but it has a very different meaning from sentence 2. He really cannot be trusted= you should not trust him. He will betray you. He's deeply insecure= he is always very nervous, and afraid that other people think bad things about him. He doesn't have much self-confidence or self-esteem. Insecure people are sometimes very trustworthy: they work hard to prove that other people should trust them.