Yes, it will. I had an optimus 7 previously and bought the wpcentral app, skulls of the shogun and some other apps and they were all available to me without having to buy again. You'll have to use the same account though
Yes and No. I had an HTC Arrive and switched to the Nokia 928. I had a number of Apps that were missing (some of which still exist on WP7 after a hard reset) and a number of apps that don't exist at all. Angry Birds is an example. I purchased the original version and it was replaced with a newer version. I would have to re-purchase it. I purchased Angry Birds: Rio instead.
I'm not sure why some Apps are WP7 exclusives, but it all depends on what's available. I've heard a number of people say by the time the 928 came out many of the WP7 exclusives were now on WP8 as well.
No I broke my HTC Arrive and bought a temporary replacement until I could switch to Verizon for the Nokia 928. My version of Angry Birds was gone. The same new Angry Birds did exist on WP7 that existed on WP8. I think I didn't actually buy them so I would have to test it. I suppose I could try to install RIO on my replacement HTC Arrive.