The o-rings I ordered arrived today. There's a little slot at the base of the eyepiece threading you can see in my previous photo, and they're a perfect fit; they extend the eyecups just enough to let me settle them comfortably into my eye sockets without major blackouts, and the action and click stops function perfectly. I wouldn't call it a comfortable view compared to the Viper HD, but that's to be expected, and I'm very happy with the result.
Now I can actually give the Mavens a proper workout, but I think it's safe to say I'll be keeping them. With spring just around the corner I had a good time watching a great blue heron and some beavers today. It's nice to have something I can easily tuck into a hoodie or coat pocket, and I don't even mind the chintzy drawstring bag since it doesn't increase the size much (I also ordered the hard storage case which is nicely made). Overall I still definitely feel they're well worth the price difference over the M7.
For reference, the o-rings are M2 x 29 (i.e. 29mm ID, 33mm OD), just standard Buna-N.