I have a question with Updated Intel. Does it trigger if a villain plays or pulls a card from his discard? This would apply with Omnitron and Gloomweaver a lot.
The card's trigger is "Whenever a villain card enters play", so both of those would trigger it.
Plague Rat has no discard effect so it shouldn't buuf on other turns than yours unless the environment screws with you meaning as long as you discharge the damage bonus by say using your base power you'll be fine (you don't need to put the damage tokens on other players and conveniently your base power is a reliable outlet unless you can't use it again environment shennanigans), it just becomes a strategical choice and yes even on advanced Plague Rat the extra damage I dealt this way made the difference between victory and defeat.
Kismet is an example, I believe, who has both discard and hero-on-hero damage, so that's a situation where you would need to be careful. And you mentioned the environment, where discarding seems reasonably common---for example, discard costs to get rid of environment cards. Even just for Plague Rat, I believe one of Parse's cards gives her a power that involves discarding---I want to say Segmentation Fault?---so that's an example. If you play Critical Multiplier early, and now everyone is infected, using that power would put you in the situation where you're required to buff someone's next damage even though it's likely going to be self-inflicted.
But the broader point is less about how good or bad specific situations are, and more about being aware of this potential downside of Critical Multiplier, since it's easy to overlook the card's potential downside in certain match-ups. Yes, if you discard with your card play, you can give yourself the buff and use your base power so that heroes aren't punishing themselves. But even that is potentially altering your gameplay as a result of having played Critical Multiplier. Like in the Segmentation Fault example---you can either use that power and potentially increase the damage a hero takes, or you can buff your own base power's damage instead of using Segmentation Fault. Either you've avoided using a power that might've helped you more, or you've taken more damage than you would have otherwise---either way, Critical Multiplier has affected how the game is playing out. It's good to know about those possibilities ahead of time so that you can make smart plays and think ahead to future turns. I'm certainly not saying Critical Multiplier is bad, or even that this is a significant drawback, just that it's worth taking note of.
But you would want to think seriously about whether it's worth letting the villain play a card from their deck in place of anything other than playing a card from their deck.
Piling Critical Multiplier bonuses on yourself is seriously unwise unless Plague Rat has already gone through all his Infections and all the cards that can get more Infections. If you do get an Infection, you'll self-damage before you can do anything else, unless someone can give you a power between the villain turn and your turn. In that case by all means go ahead, but otherwise it's really dangerous to do this.
You can discard for Critical Multiplier in your Play phase, though, say by using Recompile. Then you can just put all the damage bonus points on yourself and then use your base power to discharge it all immediately so that you don't bite yourself super-hard the next time your turn comes around :).
I suppose it would be funny if an Amulet-wearing Nightmist was in the game, though - chuck all the damage bonuses on her and laugh as the Infection damage gets redirected to Rat (assuming he doesn't have Sewer Fiend out, of course). I suppose you could do the same thing with Tachyon and Synaptic Response.
Gauge is a nice combo for card setup with Updated Intel. I have found sometimes with using Updated Intel on hero decks it becomes a bit pointless if the card on bottom isn't worth putting on top. Having both in play gives more options for setting up any hero deck.