In regard to 1-Resistance to Ketones, etc.- HDPE is more resistant to ketones, etc. This is because PVC is soluble (at least to some extent) in ketone, aldehydes, etc. Cyclohexanone and THF are two common solvents used to dissolve PVC for lab testing.
As to 2- PVC doesn't absorb much in the way of water (my experience is with medical PVC applications, not waste water applications). Then again HDPE won't either. I wonder if it's a difference between absorbing little water or very little water. However, many PVC compounds include plasticizers, which may absorb water.
PVC degrades by forming hydrochloric acid and conjugated alternating single and double carbon bonds on the polymer backbone. The free hydrochloric acid speeds up the dehydrohalogenation, generating more free HCl, which further speeds up the degradation, etc. This is why PVC needs stabilizers and is prone to oxidation.
Hope this helps.