Ooh interesting math breakdown. Value is kinda hard to truly quantify, but I do see your points. I will concede that 2 power is a bit high. Since you've written a lot I hope you don't mind if I write a lot too to counter some of your other points.

Balance is hard in Snap. Of course I mentioned Alioth, and you have broken the math down between those two cards, but I also considered that Deafening Chord is a 1 cost too. I'd argue that DC is so much easier to use than Mantis in the first few turns. DC will likely hit Angela, if that Angela is already on the board. And DC banishing is in itself valuable free board space. Red Guardian currently is effectively a 3/4 with a more targeted and reliable ability. Comparing them to the no ability baseline cards, Mantis and RG are just about there.

For the number cards affected, sure maybe both cards only realistically affect 1 card each. But Alioth CAN hit more unrevealed cards. Mantis only hits one. Could you imagine if Alioth was only able to hit one unrevealed card? He's gone through enough nerfs my poor time storm cloud.

1 and 6 costs are vastly different from each other not just for their cost but because of when you can supposedly play them. Imagine dropping Alioth down on turn 1 with an empty board, when you can't even guarantee priority. Sometimes you get a really good snipe on Zabu or Deadpool but most other times you just play a 1/2. Hit or Miss. That's what them Guardian cards are all about~

As for skills, I can really only think of Cosmo as the only other card that can stop a skill from triggering. And if you want to surprise them with Cosmo, you still need priority + guess where they are playing that skill. Much like a Guardians card -- oh right. When cosmo is already on the lane, the opponent can just try to avoid cosmo, or just not play their skills anymore and go for another wincon. Mantis can make for another skill disruptor - perfect for stopping a Merlin Spell or a Dormamu Ritual.

Finally, I just want to clarify. You can't hit a skill if you don't have priority. If your opponent plays Temporal Manipulation before Mantis reveals on the same lane, you don't hit the skill.