To answer your question, if 51% percent of the miners started creating 2MB blocks then Bitcoin would have effectively increased its blocksize limit.
If it was only 51% (and why not just say >50% as there is nothing magical about 51) then it would be very problematic as has already been pointed out that could easily be reverted (as you have to continue to control the hashing power).
This is why any hard-fork should require a lot more % of the hashing power (it costs miners a lot in electricity to create new blocks so they are not going to want to be creating invalid ones).
If a >50% attack like this happened then there would be two Bitcoins and you could literally spend your coins twice. IMO if that actually happened then Bitcoin would lose 99% of its value (which is most likely exactly what the banksters are hoping to achieve).
First thing you said here is accurate, it would indeed not be a good idea to fork the network at 51%, however theoretically this is possible. Having a higher hashrate like seventy five percent is better, which is what Bitcoin Classic is now aiming for.
The second point you brought up is false. After the split it will not be possible to spend your coins twice on the same chain. The chains will have effectively split creating two separate currencies, it does not make double spends possible, for you to say that is just baseless fear mongering.