Congrats on posting your first monster.
1) Agree with all that have been said.
2) Monsters don't get 'daily' powers. They get encounter powers. This is because a monster is only going to be in 1 fight, so it won't have the opportunity to use something in a day's span.
3) Given that it's an artillery monster, it doesn't need the double attack, really, unless you want this guy to get into melee. Artillery monsters (in general) want to stay the heck out of melee, since they have little HP or AC or movement capabilities. Now, if you want to change that (i.e. many controllers do so in the middle of melee), go ahead, just realize that you need to do a little gymnastics. Artillery monsters have HIGH to-hits, but don't do buckets of damage or have high defenses/HP, so they need something to counterbalance this.
4) I like the blinding blast thing. Although giving a monster an AT WILL area affect that blinds? That's harsh. What I would do? Switch the damage and frequency of Icy Barrage and Blinding Ice (so Blinding Ice becomes an encounter power that does 2d8+4 and Icy Barrage does 1d10+4 and is at-will). Then I'd let Blinding Ice recharge when the Mephit is bloodied. This way, the Mephit stays back, if someone charges it and attacks it in melee, he blinds them and runs away.
5) Instead of thinking of summoning as an ability, think of summoning as an abstract explanation. An NPC can "Summon" his allies outside of combat (i.e. just as PCs kick in the door, or moments before hand, or they appear a round after the battle has begun). This way, you can explain "Where did these monsters come from?" or "Why are they hanging out together?"
In other words, there's no difference, in game terms, between 4 monsters being in the same room when the Encounter starts, 1 monster yelling for help for the monsters in the next room, or those same 3 monsters appearing from Beyond. Either way, you're going to have an encounter with those 4 monsters; describe it otherwise how you see fit.
6) FYI, there are some
mephit conversions here (Steam, Mud and Salt). The same guy
has done a bunch of conversions; I swear by him.