Main wallet unspent transaction

I have a question about an unspent transaction, we know that after some transactions we have a change address created by the main address, now imagine that Bob mined 6 BTC, now he transferred 2 of the two Alice and now in Bob's change address there is 4 BTC and they are unspent, now the question is that if I try to get unspent transaction of the main wallet with bitcoin core that I imported the address of Bob's main wallet, will it show me 4 BTC? or it can show if I try to get with child address?

Another question is that, is there any way to find a parent or main wallet, child keies?

All wallets will calculate and show the same amount as the "balance" for an address. In your example, all wallets would show the balance as 4 BTC. There are exceptions if a wallet is faulty or is not synchronised (not up-to-date, doesn't yet know about latest transactions).

If you import an address into a wallet, it will show the correct balance but it will show that amount as "watch-only" or "non-spendable".

There is no way to discover private-keys from addresses.