The invalid checkpoint error is on purpose - This puts your wallet in safe mode so you cannot fuck it up further. THIS IS TO PROTECT YOU FROM YOURSELVES
Follow my instructions, leave your wallet open, and STOP FUCKING WITH IT AFTER THAT
I'm not going to keep repeating the same things. Luckily all these errors are there for a reason so you guys cannot totally fuck yourselves, because quite a few of you would have already corrupted your wallet.dat files if the errors you are bitching about were not present
The chain will fix itself; Follow my instructions, leave your wallet open, and STOP FUCKING WITH IT AFTER THAT
I seriously do not know how to make it more clear