I did not compare the MDF weight, but it seemed pretty heavy, if I had to guess as heavy or heavier than plywood.
I suppose the 1.6's would work with the OB's sideways as a base. One concern I have with older Magnepans is delamination and deterioration. I once naively purchased a pair of smallish (don't remember the model) Maggies for $150, and the seller told me one didn't work. He said Magnepan would provide parts to repair it. When I got them home and took the failing one apart, the membrane was dark brown literally flaked apart as I touched it. When I called Magnepan the cost to replace the panel was close to or more than the MMGs. I ended up giving them to a buddy who bought a turntable from me. I have no idea what he did with them.
Since my crossover designs are fairly simple first order affairs, I use the L-Pads to adjust the levels of the various drivers to balance the volume of the tweeter/midrange vs the woofer which is usually lower.
Magnepan changed their adhesive in IIRC 2005 after an R&D effort to find a more stable adhesive. The ones made before then are moisture sensitive or in the very early years sensitive to ultraviolet, but the ones made since then don't delaminate. So what I'd do is get the serial numbers of the Maggies in question, then call Magnepan's service department. They'll tell you when they were made and if they use the newer adhesive.
You can repair delaminated Maggies yourself, by the way -- Magnepan sells you the wire and glue for a nominal price (they can't ship the glue in the winter, though, since it freezes). It's a weekend project -- you remove the fabric socks, take off the old wire, foil, and adhesive with acetone, put new wire and foil on with 3M spray cement, then lightly brush the adhesive over it. But of course if you have to do that, you should pay less for the speakers.
By the way, I heard from Wendell Diller, Magnepan's director of sales -- he's the guy you spoke to. He remembered your call and saw your post on the Asylum. Wendell said he thought that the .7's would work well in your application. The problem with the MMG's from his perspective is that the midrange drivers aren't fast enough. Remember that the MMG is a budget speaker -- Wendell referred to it as a Volkswagen. It offers incredible bang for the buck, but it's entry level and it isn't suited for every application. So rather than couple an entry-level speaker to a pair of OB subs I think you should either go for the .7's, which are maybe $300 above your budget, or buy used 1.6's. The .7's would be great because they use the latest technology and they're really compact -- with the GR woofers you don't need all of the bass diaphragm area of the 1.6.
If it were me, I'd go with the .7's, they've gotten rave reviews.