I have not seen that Haifa Oil Refinery has been hit. Where are you getting your reports from?
The issue with the firing rate is that it would make sense after Israeli defenses have been exhausted to which point at least 5-10 of those daily 20 missiles will get through. Right now they all being shot down. Palestinians live all over Israel, including many in Haifa so we would see and report any direct hits. We are mostly only seeing interceptions at the moment.
Let's say this war goes on for two more weeks, there would not be a cost imposed on Israeli's if it wasn't for Hezbollah intervening. If it goes longer than two weeks, with current Iranian rate of fire, it's possible US moves Patriots/Thaads from other locations to Israel to replenish them.
It would make most sense to dedicate one day to force Israel to use up many interceptors. Maybe using cheaply designed missiles with small warheads just for this purpose, in the future at least, if not possible now.