Also, will my wallet.dat hold the private keys of all my regular addresses? Even if I used the wallet.dat to generate let's say 5, 10 or 25 thousand regular addresses?

And I suppose that if I have the public and private key of a regular address, I can always regenerate the segwit address and spend all funds that are on that specific segwit address, right? Will the "bitcoin-cli addwitnessaddress 1myregularaddress..." command always generate the same segwit address?