1. I noticed that backup file lenght has changed, it some 100 bytes bigger that previous keys. Is that Ok?
Thats absolutely fine. This may due to any additional parameters (new addresses, ..) recorded in the backup file.
Or due to a new format.
2. Do I need to also restore BACKUP1 (probably dont)?
I guess you are using a HD wallet (12/18/24 word seed phrase).
In this case no. Since your backup contains "all" addresses of this wallet. It automatically creates new addresses, based on your seed, on demand.
3. I noticed, under my addresses, there are 4-5 adresses (and more under Old). What that means? Are all addresses mine?
Yes, your (probably) HD wallet creates a new address for each transaction. Your seed and a "counter" is used to create a new address.
This is done for privacy reasons.
4. Another person also have my initial backup, BACKUP1. What happens if he/she restore it to their wallet? What can I do to prevent that? Can I generate new address (using same wallet app already installed) and transfer all bitcoin holding to that address, will it prevent this case? I am not so familiar how wallets work.
If someone has access to your old backup (BAKCUP1), he also has access to the seed used to generate this wallet.
Therefore ANY further generated address using this seed is not recommended. This wallet should be treaten as compromised.
You should create a new wallet (new HD Wallet, not just a new address inside your wallet) and transfere your funds to this wallet. At this point you will have
to write down your seed phrase as this is your new backup.
Some wallet apps provide the option to create new HD wallets within the app.
5. Can I now backup my keys in new file and then delete all previous backups and my balance will be ok? I will not do that, but just interested do I need all previous backups or the last one is enough.
"Only last one." - But you should never delete old wallet/backup files.
You should backup your seed as a word phrase. I would not recommend to store backups digital only.