If the network suddenly splits into 3 sets of 33, then no future transaction can gain a majority, as 33 votes is always less than the 50 required, thus the ledger in all 3 groups is never appended to. Those transactions time out and no forks results.
I cant explain it in any simpler terms than this, and if you still believe that there will be forks, well, I dont know what else to write.
What about long range attacks? Is the evidence of past voting history difficult to forge?
Endorsement information is included in transaction data, which is signed by the transaction creator. They are not malleable in any way.