Bitcoin core addresses are all empty? My PC, got burned entirely in a fire at my warehouse.
Luckily, I had create a backup of my bitcoin core  wallet.dat > 2 days before ! &lt; ,so I bought a new pc, with Windows 7 on and now I'm downloading everything from the beginning.
In the 6th day, as I had 21 weeks left, my wallet popped up the message: 
Number of blocks left : Unknown Syncing headers
So I believe, that it might take a lot longer, since it might need a -reindex
or a -rescan.
So I read about the article on migrating Bitcoin core to Electrum.
when I searched for the receiving addresses that had money in them, I found only a small fraction of the total amount in blochchain.info.

My Questions are :

A) Will I need a -reindex or a -rescan FOR SURE ? and what's the difference of these 2 ?

B) Why my addresses are almost empty ? OR why can't I find the addresses which have the correct balance inside them ?

Thank you in advance.
A) While downloading entire blockchain, Bitcoin Core sets indexes and scans the transactions which belong to your wallet so no, it does it automatically. This functions exist to allow you to check the balance after importing private keys from outside manually (as default, it rescans after each private key is imported but you allowed to do a rescan after importing multiple private keys). reindex basically refreshes the table which contains transaction positions in the disk to search a transaction fastly with a given TXID. rescan scans entire blockchain to find transactions which belong to your wallet.

B) Sorry, there is no proper answer for this part. After creating backup, did you receive any amount of money into new addresses generated after the backup? Or, did you send any amount of money to anyone (new change addresses might be generated)? What was the keypool value? Its default value is 100. If you didn't change it and if you did not send or not receive any amount of bitcoin more than 100 times after backup, you must be able to have all the balance. Did you check all addresses you have in the wallet on blockchain.info? Also, are you sure your wallet has not been compromised? If you are not sure, check the transactions of your addresses in blockchain.info and try to remember your last transactions then compare them. If you see any transaction which you did not perform, then your wallet backups (or your private keys) have might be stolen.