I don't understand the problem, really.
Let's just say that once you build a moat that connects to the volcano invaders won't be able to climb over it.
The volcano is too steep for climbing and no moat will stop flying enemies.
Your proposed setup, Dariush, would need double walls to be build and magma pumped in the middle, which is okay, but this would limit the space right from the start. I like 10ebbor10's plan in the respect that it can be modified and it will give you a bigger project to work on with the option of always upgrading it. Magma can still be pumped into the moat if necessary, so you won't miss out on it. And it will be something to look at, which is always something I'd favor.
You can always build on and around the volcano, go for a giant dorf with glowing eyes that can spit magma or whatever, but just let's go step by step here and see what would be the easiest setup to begin with that will give you the option of building further in the long run. I understand the sentiment of going full magma as soon as possible, that's why you chose the volcano, obviously. You just lack a lot of equipment that would be necessary to make this a safe construction or rather to make your plan work in the first place. If you start building walls and moats, you can always flood them later, maybe build your retractable stairways, and wall the moat off from the other side. I don't really care. But I don't see this discussion leading anywhere right now, since 10ebbor10's plan does NOT contradict yours.
Edit: And it's hard to find something that everyone is okay with, I can also see the problem there. I don't want to push you in a direction, really, just consider that this would be an operation that takes multiple turns and that other players have made suggestions that would expand the usable space outside. You can still put traps indoors, outdoors, everywhere. And if you build a moat it will not damage your underground construction, let's say Donut dug deep enough. Would be really counterproductive otherwise.
DoubeEdit: I'm also on your side, or rather the Dwarven side, although I want to see some action. I decided to leave the villany to other people, in some respect, but you will see about that later :3
TripleEdit: The two weird shapes in the snow are obviously snow angels. How could you not see this?