HFS isn't really that bad if you can survive the initial wave. I don't think it even murders your FPS too much once the initial wave is over, as the pathing behavior is a little like a wild animal's on the subsequent clowns, some of which can't even fly, so they can't come out of the HFS to attack you, anyway, and like wild animals, they only respawn as you kill them, and only chase after dwarves if they see them, unlike the initial wave.
Therefore, breaching is terrifying, but surviving and having a decent fort afterwards is possible. (Note: surviving the first wave is HARD. Either have 50 warriors with legendary skills, divert the HFS so that it attacks a goblin seige to soften them up before your warriors fight, or build some kind of giant chamber of spikey doom or obsidian hammer shoot. In other words, you have to make SERIOUS preparations to survive this.)
Oh, and keep in mind that now, there are adamantine veins in every 2x2 portion of the embark, so you have 4 adamantine veins in a 4x4 embark to explore.
When mining the adamantine, you are actually probably safe as long as you don't go into the magma sea-levels of the adamantine. (I don't think you can breach until you are at least a couple levels into the magma sea.) So long as you aren't getting cancellations because of warm stone, you should be fine.
As for what to MAKE with the adamantine, I'd honestly reccomend armor. Maybe a spear or a sword, as they have terrific edges, (but blunt weapons are useless) but a breastplate of adamantine is still terrific at preventing piercing attacks, although it should be backed up by some padding chain or leather made of denser material.