1- is it possible to generate a RAW transaction even if the wallet is in offline mode and not synced?
Yes, as long as you know which transaction outputs you want to spend.
2- Wallet B online can spread the RAW transaction provided by someone else?
Yes.
3- while the wallet A is offline, if I spent some BTC from another online wallet and using output that I want to include in my RAW transaction, the RAW transaction is generated correctly, but will be invalidated (attempt of double spent?).
Correct, if you have already spent the output in a transaction that is already confirmed then all peers will reject the new transaction as invalid due to the attempt of double spend.
4- Security issues?
Keep your keys safe and don't share signed transactions that you don't want broadcast.
Is it possible to generate offline transaction with a mobile wallet (Ie. Android app)?
I'm not aware of any mobile wallets that have that ability built into them.
thanks
You're welcome.