I don't want to necessarily do this, but what prevents this scenario from being possible? [just timestamps and a difficulty modifier alone? - if that's the case then why can't I modify the client to be set at a much higher difficulty to mine stuff in anticipation of a harder target as opposed to simply using network difficulty?]: How come I cannot make a modified version of Bitcoin QT and all historical transactional data re-utilized to mine all past stale blocks and simply premine all remaining bitcoin without difficulty?
Depending on what you meant to ask, the answer is one of the following:
If you run a Bitcoin-qt fork which is incompatible with the rest of the network, you're running a separate currency which has no value (because no one else accepts it).
If you ignore blocks and forge historic timestamps to create a branch with more blocks of low difficulty, it will not be chosen over the main branch because the choice is made based on total difficulty, not number of blocks.