1. Hmm. Good question. Based on what we see on Earth at diverging plate boundaries, I don't think you are generally going to get the kind of explosive "disaster movie" type volcanism you see from volcanos created in subduction zones like around the Pacific Ring of Fire. But then the mid-Atlantic Ridge, where the North & South American plates are moving away from the Eurasian & African plates, is the longest mountain chain in the world. When measured from the sea floor base, the ridge rises some 15,000 feet. But it's hard to be impressed by mountains (or volcanos) that are mostly hidden beneath another 8,000 feet of water.
And again, as far as the hotspots go, you can really put them anywhere, they don't need to be near plate boundaries.
2. I think I lost my train of thought briefly when I was taking about your plates and just got focused on what direction they were going. I was comparing the three sets because the pairs were moving in unison but, as you say, there weren't any mountains between two of the pairs, as there right oughtn't to have been. So, Kudos for you. My bad for the confusion. /smack
B, C & H coming together should develop a very high mountain range. However, since all you're dealing with here is continental crust (not as dense or high in water content as oceanic crust), there probably won't be a lot of volcanos, just really really huge mountains (like the Himalayas). If volcanos are a must-have you could stick a deep inland sea in there somewhere next to where the mountains are forming, it being the last remnant of some ancient ocean soon to disappear forever (like the Mediterranean).