Mining pools can be algorithmically decentralized, but you will never see this in Bitcoin.
Mining can be decentralized with cpu-only PoW, but you will never see this in Bitcoin. Scrypt is not cpu-only.
Checkpoints are a precaution while the network hashing rate is smaller than for example Google's server farms, but isn't needed after that.
Cpu-only will reach that point much faster than Bitcoin did.
Soon.
If you make a CPU only coin and assuming it remains so over time, you will have huge botnets controlling the coin. It doesn't make it any better.
The idea in the OP sounds promising, even though I have quite a few reservations. Of course, its still a theory now and unless we see it working in practice we won't know if it is any good.