1) If I check a blockchain explorer, I see that some blocks are mined very fast and others not. Some blocks have only 100 transactions in it
Miners are free to pick any number of transactions they want as long as the block size doesn't exceed 1 MB.Edit: I forgot to mention that witness data are not counted in the 1 MB limit and any block can include up to 1 million virtual bytes or 4 million weight units of transactions.
2) It takes approximately 10 minutes per block to be mined. If I make a transaction, my transaction is validated before a new block is mined ? If not, do I have to wait 10 minutes between 2 transactions ?
10 minutes on average, not exactly 10 minutes. Your transaction will have the chance to be mined in the next block. Depending on the fee rate you set for the transaction, it may take more time to be confirmed.
2.1) Every information added in a bloc that will be mined change the hash. Is some transactions added in a block DURING the time the miners are working to find the right hash ?
Miners make a candidate block and can add or remove transactions from the candidate block until the block is mined. 
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