What would happen to me if I have a possibly tampered Bitcoin client and a network with more malicious nodes than honest ones?
This would count as forking the chain. A normal/honest client simply wouldn't accept any chain other than the one with the official checkpoint.
Also with PoS what's really important is only the developer's signature, however many people you have to sign the checkpoint it's only the client signature validating them to be true.
Again, the signature for the checkpoint was just a convenience. It just saved downloading an updated client.
A better way to think about it is that there are 2 clients.
The "beta" client is a POW based client, but it won't accept any blocks after 70000.
Once 60000 blocks have been found, then 50000th block becomes locked in and a brand new "release" client is created.
The new client has the 50000th checkpoint hardcoded. This is no less vulnerable than bitcoin, which has the genesis block hardcoded.
If someone sends you a client with a fake genesis block, then you will mine on their (fake) bitcoin chain too.
The only POS coin running at the moment has a developer based checkpoint system to prevent roll back, but that isn't strictly required.