if I have a an old wallet.dat which had transactions lets say from 2017 and I have my bitcoin core fully synced and up-to-date, so simply replacing the default generated wallet.dat with that old one will be enough (while it's offline) ?
It should be as easy as that. If it doesn't happen to work then open Console in Bitcoin Core (Window -> Console) and type in
rescanblockchain- can I do all the following (totally offline) ?
Yes, you can. Your transaction will be broadcast once you connect your main PC to the Internet and let Bitcoin Core catch up with the blockchain.
- hopefully you have any answer for my Wasabi question, if no, no problem ?
Unfortunately, no. Wait for ETFbitcoin's reply.