It IS stable. It does not randomly burst into flames, at least not unpredictably.
I don't know what you're thinking of when you say stable, but I don't think you'd want it if you understood what a truely stable explosive (A la C4) requires to be detonated.
Yes, Kashyyk, but projectile travel time is increased, and as you certainly understand the basics of angles, you can realize that long range can either be good elevation, good propellant, or both. They've got a bit of elevation, but they can't physically traverse their largest cannons high or fast enough to hit out faster Skyskiffs. Upgrading the Alsamma with the armor required to absorb some of these hits will prevent them from getting shot as they run in. Or, you know, we can just sorta fly above their range. They might be able to send shells on a relatively flat trajectory against almost-stationary targets in a mass bombardment, but they can't shoot down fast-moving ships, even at ranges and elevations they can hit, because it's too hard to accurately predict shell flight versus target motion plus travel time.
Well, Taricus, when they do that we'll be in a bit of trouble anyway, since we can't physically explode enough stuff to kill them all, we're reliant on the temperature and crap. THEN we go all-out on a new, MUNDANE (Easier to produce, more readily available, usable without wizards) raw explosive... Or even better, use wind magic. A single burst isn't the most difficult thing to do. If you hyper-magically-pressurize a specialized adamantine vessel with simple air, and then used a spell to super-harden and then fracture the adamantine, we could probably put some shards through a lot of Arstotzkans.