Nice work so far.
Those are my tiles on the 6mm Wargaming site and i'm about to upload part 2 of the article in the next few days. Anyway heres some pictures of mine finished


I've found the tiles very good and quite durable (so far) but yours sound different as mine dont have the foil backing. I did partially cut a couple of the dovetail bits by mistake which made them a lot weaker so I pinned it and used some PVA and it seems pretty good now. I figured that it the dovetails bits don't handle the rigurs of wargaming I can chop them all off and the boards willl still fit together, however I like them and it stops the boards moving around if they get knocked.
BTW I used "No more Nails" a builders glue, to fix the tiles onto the MDF to avoid the warping you mention. I piled all the pieces of spare wood onto the top and some boxes of figures and left each board for a minimum of 24 hours while the glue cured. It took a couple of weeks to glue all 16 boards but I didnt want to take a chance.
Any questions let me know
Cheers
Kieran