Who is we?
I meant the party which in the above scenario wants to defend Bitcoin against the mentioned attack.
Of cause you can start several btc chains. But this weakens the system.
Each chain can be attacked individually, being smaller drowning it is much easier.
I do not know. Maybe it depends on the exact attack and defense modes and all the remaining assumptions we make on the attacks.
For example: Let us assume we have 100 different chains and a centralized attacker (let's say the Fed) owning 51% of the hash and attempting an attack where he never adds transactions to the chain. Initially, the chains are all exact copies. With 49% of the hash and with 100 different chains there is a non-negligible chance that the good guys might at least in 1 or 2 of these chains win the block earlier than the attacker. So - under these assumptions - I am not sure if this claim is correct. I would agree, if attack modes also comprise double spending attacks (but still, I do not know - for lack of an appropriate model).
AFAIK we do not have the proper abstract models in place yet to be able to calculate and derive all the answers...