I also have a glass-top stove at home. So far, I really like it: the heat is very even and there is good heat transfer, provided the pan makes good contact with the surface (one skillet I have has a bit of a lip, which makes it heat more slowly).
I've slapped skillets and pots full of water around on mine, and have yet to break it. I'm probably more careful than I would be on a metal range, but it seems sturdy.
But to your points:
1) There is no way cast iron could melt the glass. Iron melts lower than does glass (1200*C vs. 1500*C), so before you manage to melt your stove, your pan will be a puddle. It may, however, be possible to deform the stove top if you let an empty pan heat for some time. I don't see why this would be more of an issue with cast iron than with any other piece of cookware.
2) A heat defuser will work fine. Don't get the kind that is designed for use with gas, but anything else should work fine.
3) The burners on my stove are smaller than my skillet, so I find I need to move the skillet around to heat the edges. Also, flat-bottomed pans and pots seem to be much more effective on the flat glass top, as conduction seems more effective than radiation here. I polish the bottoms of my pots to get better heat transfer, although this is just me being anal.
Also: cast iron can't scratch glass. Glass is much harder than iron (see this wikipedia article), glass having a 6-7 on the Mohs scale, iron having a 4.
My name is Tom Wirt, with Clay Coyote Pottery. I'll try to shed some light on the intricacies of clay cooking pots, especially tagines.
You can use any flameware tagine directly on the glass stovetop. This includes, Emile Henry, Le Crueset, and Clay Coyote flameware. These are pots with either a metal base (Le Crueset) , or a type of ceramic called flameware (Emile Henry, Clay Coyote which is formulated and made to take direct heat.
Normal stoneware clay pots and earthenware pots will not do this. Stoneware should never go on a direct heating source, gas, electric or glasstop. It will crack with or without a diffuser.
Earthenware ceramic pots, typically identified by a reddish clay color and some absorbency by the bare clay (typically the bottom), do need a diffuser and should be started over a low heat. They can crack if used over sudden or too high a heat. Remember that these pots were originally used over charcoal fires.
Metal, obviously is fine.
The flameware ceramic pots, are designed for direct heat and are actually especially good on glasstops as the top spreads the heat better than electric or gas.
Clay is a insulator, not a conductor. Thus the heat doesn't spread much, but, with a highly liquid food like a tagine, the liquid spreads the heat. Basically a tagine is cooked at a simmer, even though the pot would take the heat.
Induction stoves require a metal plate with ceramic cookware to turn the electromagnetic force into heat.
You can find more info on my blog.
Best Answer
A high heat silicone adhesive will do the trick. Something like this (obviously only need to buy one, not a whole case).
Also, I've had some luck with oven door adhesive (though never tried on cookware) and auto adhesive.
All are oven safe up to 400-500F continuous, so you should be good for baking.