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.
It depends. Traditionally cast-iron did not come pre-seasoned.
Lodge, as you know, specializes in pre-seasoned and enameled cast-iron pans. Lodge pans work great, but there isn't anything exceedingly special about their seasoning; it's simply vegetable oil (soy) baked onto the pan. You should be able to achieve a similar seasoning quality at home using the methods described here:
What's the best way to season a cast iron skillet?
Wagner skillets do not come pre-seasoned from the manufacturer. However, they did typically come with some coating in place to prevent rusting. This coating, while safe, is not intended to be a "seasoning". It wears off over time, and it's role is subsequently filled by proper seasoning.
Update
Based on your photos, that looks to be the coating I described above.
Best Answer
It should work just fine. Preheat your oven, with the griddle inside, for at least an hour. Build your pizza on a peel that has been sprinkled with some cornmeal so that it slides off. If you don't have a peel, use the back of a sheet pan and some parchment paper underneath. Then slide the pizza and parchment onto your griddle.
If you have a new oven, you may need to go through a preheat/burn-off without food first...check your manual.