They are valid on the main fork.
Even in the case of a very swift attack, as it was mentioned, max reorg depth in NXT is 720 blocks, that's about 12 hours, there will be many transactions that will need to be censored because they depend on transactions transferring funds from the attacker's addresses. And, if you ask me, I can't imagine how it's possible to collect priv keys for stake enough for such attack in such short time frame.
The attack scenario sees the main fork being orphaned.
Almost all attacks see the main fork being orphaned. Thank you for stating that, captain.
Whether you can't see how its done or not is kind of irrelevant. The risk is there, the cost is virtually zero and chance of success is very high IMO.
"IMO" - yes, this is your opinion.