None of what you wrote is an accurate description of how Lightning Network works.

Furthermore, A would have to be pretty STUPID to think that he can chargeback by broadcasting tx1.  It should be obvious to A that broadcasting tx1 will not help him at all.  He has already sent the 10BTC to B and he has no way of getting that value back unless B is STUPID.

If you are engaging in a transaction with someone that you do not have a trust relationship with, you need to take actions (BEFORE YOU SEND THE TRANSACTION) to protect yourself.  As an example, perhaps use an escrow provider?