That entire perk tree is out if you're looking for dual stealth. Daggers don't benefit.
Daggers do benefit from the perks running up the middle of the One-Handed tree (I know they get the +x% damage ones and I assume they should get the power attack and dual-wield-related ones, but haven't tested). It's just the axe/mace/sword ones that they don't get.
Swords counts for daggers too, right?
It specifically says "swords", not "blades". A dagger is not a sword, so most people seem to be assuming that it does not. An easy way to test would be to get that perk, then fight, say, half a dozen battles using daggers only. If you don't get any "critical hit!" notices in the upper left of your screen, then, no, it doesn't work with daggers.
To the OP:
For sneak attacks, the best thing in the world is daggers with the x15 sneak attack perk from the Sneak tree. Failing that, I suppose that maces would be your best option from within One-Handed, since they have the highest base damage, their "ignore armor" perk prevents that damage from being reduced, and a sneak attack is all about doing as much damage in the first strike as you can. It doesn't matter if maces are slower to get in their second shot because the enemy should already be down after the first one.