Hollowing out spires is a very, very bad idea (they're already hollow, in fact). On every z-level of a spire, there's always a central 4x4 core which leads down to hell. If you can figure out where that is, you can mine any bits hanging off it safely. For instance, it could look something like this:
..........
....OOO...
.....OO...
...XXXX...
...X X...
...X X...
...XXXXO..
....O.....
....OO....
The Os would be safe to mine out; breaching the Xs would release the clown car. Any two levels of the spire have to be connected on the inside, so you can use the layout of one layer to determine where the core of the next one is.
Typically candy spires in the magma sea will extend upwards into the rock a ways. Just carefully dig a shaft right above the sea over to where the spire is and hopefully you'll have a good number of layers in the rock. Getting candy out of the magma itself is a bit of an undertaking, and probably not worth the trouble. If you're lucky, spires can pierce all the way up into the lower caverns.
You're right about blunt candy weapons being horrible. In terms of armor, adamantine should be fine for all purposes. Just make sure you make breastplates and mail shirts, because bolts pass right through normal armor. Addy works well for shields because it's so light, but you may want to use a heavier metal for those, in case they decide to bash with them.