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.
You'd then have to force everybody to download your forked version. And nobody would do that because it contains malicious behavior.