Swarm Concealment power issues

I’m trying to use a swarm of insects as a Concealment effect, making the insects flying around the enemies and not let them see or hear (due to the buzz sound). but I faced some issues.

Concealment has a Personal range, so it will only work on me. To affect others, I should use Affect Others or Attack. Affect Others make it possible for others to disable the effect at will, which is not what I have in mind. Attack is imposing, but it has a problem: the power does not work on me anymore. I would need to buy Concealment again and maybe link it to use both versions of Concealment at the same time. A ranged or perception range should adjust the power to not need to touch in close range the others.

But here is the issue: Concealment controls who’s been concealed, but not who’s gonna have their senses affected. The concept of a swarm to block senses lies on this premise, that you choose who is going to be affected by the swarm and not who is going to be concealed. In other words, Concealment concealed some people from everyone, but my effect seems to be the opposite: you blind the senses of some people to prevent them from sensing everyone.

How can I build such power effect?

Does the “Pierce Magical Concealment” feat allow a character to see magically concealed characters?

One of the players of the oneshot campaign that I’m writing chose the feat Pierce Magical Concealment (Complete Arcane, p.81), as a DM I’m not sure how it would work in an encounter I am planning.

The manual states about the Pierce Magical Concealment feat:

You ignore the miss chance provided by certain magical effects.

Your fierce contempt for magic allows you to disregard the miss chance granted by spells or spell-like abilities such as darkness, blur, invisibility, obscuring mist, ghostform (see page 109), and spells when used to create concealment effects (such as a wizard using permanent image to fill a corridor with illusory fire and smoke). In addition, when facing a creature protected by mirror image, you can immediately pick out the real creature from its figments. Your ability to ignore the miss chance granted by magical concealment doesn’t grant you any ability to ignore nonmagical concealment (so you would still have a 20% miss chance against an invisible creature hiding in fog, for example).

This doesn’t specify if a character with this feat can actually see an enemy that is, for example, under the effect of the spell Invisibility (Player’s Handbook, p. 245).

I am planning an encounter in which the party will be invited by a wizard for dinner, an assassin under the effect of invisibility will pretend to be an Unseen Servant (Player’s Handbook, p. 297) until the wizard gives him the signal to attack the party.

Will the character that has the Pierce Magical Concealment feat be able to see the assassin, or at least have any advantage in noticing he is not an Unseen Servant?