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, open Console in Bitcoin Core (Window -> Console) and type in
rescanblockchain after you load your backup and confirm that its balance is wrong.
- 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.
By the way, how do you intend to spend your coins from the guest machine?
here what I am thinking about
I have a virtual machine with bitcoin core installed on plus I have bitcoin core installed on the main machine
being totally offline
I will create several offline wallets from the virtual machine and send the bitcoins to them from the old wallet using my main machine as being some how paranoid here
regarding mixing, I am first timer into coinjoining, so I will read more about post-mix on how to keep the anonymity the same or with at least, least degradation possible.
Most probably i will go with wasabi due to high anon. set