Those are some good examples, you're right. It is quite a bit less regular than I remembered from my high-school days.
For 'er' though, isn't the rule essentially that it is pronounced 'a' at the end of verbs, and 'ar' otherwise? I realize that is not a phonetic rule, though.
Unfortunately no. See e.g. boulanger, cahier, etc. (Although I do believe you're correct about verbs, I wasn't able to think of any verbs that pronounce "er" irregularly).
I don't know of any regular, even semantic, rule off the top of my head that covers when it should be pronounced which way for non-verbs. I wonder if someone more versed in French etymology could find one.
For 'er' though, isn't the rule essentially that it is pronounced 'a' at the end of verbs, and 'ar' otherwise? I realize that is not a phonetic rule, though.