How can one blockchain with the same block heigth as another, have more Proof-of-Work
How can 2 chains with the same length have different cummulative Proof-of-Work? Is it because they might include block that were mined with different difficulty targets?


That is correct. The bitcoin protocol actually follows the chain with the highest accumulated proof-of-work, not the largest block height. 

I believe the original implementation followed the largest block height, but that left the network open to certain attacks, so it was soon changed to largest accumulated proof of work.