a) just like there's DVD-R and DVD+R, there's DVD-R DL and DVD+R DL, they're competing formats with some different characteristics: DVD-R DL has slightly less sectors (i.e. less space) available and it has the layerbreak at a fixed position, where with DVD+R DL you can set the layerbreak to your liking as long as L1 isn't larger than L0.
there's no way to change the layerbreak on DVD-R DL media simply because the standard doesn't allow it. And your version of ImgBurn is old.
ImgBurn knows all of the above mentioned limitations and it'll take that into account when you're building a DVD-Video disc, so you don't need to worry too much about it. It's also worth noting that DVD-R DL is an inferior standard, and as such, it didn't get many attention from burner manufacturers nor DVD readers, so you might run into some trouble with it in the future.