You're being absolutely played. Here are a few points (please excuse grammer / flow, I'm spitballing and very tired - I've made an account and am staying up late just because this post has red flags all over it, and I think you're being played) which hopefully show that your wife is likely lying about something.
My wife Ubers to the house, shows up and the after hours party never came to fruition. Basically, nobody showed and it was just the two guys hanging out on the couch.
In most cases (there can be exceptions) the guys would have known the chances of a party were slim. They either lied to your wife to get her to go with them (ie. had an agenda - to bang), or your wife wanted to go anyway to a house with 2 guys, on her own - or at best felt comfortable enough being with the 2 guys to meet a load of random people. Either way, she must have been pretty wide awake and buzzing to want to keep going - and presumably not really too drunk because otherwise her cousin would have likely pushed for her not to go. Or, let's say she was drunk, this means the guys wanted to take your drunk wife home with them.
At which point she cracked a beer and promptly passed out on their couch immediately.
Earlier your wife was awake enough to really want to go to an after-party she knew probably wouldn't happen, or would involve her only knowing the 2 guys she was with, implying interest in them as friends, is suddenly overcome by tiredness and falls asleep, having completely lost all interest in hanging with her 2 buddies. Let's pretend she was naive to fact that the party wasn't happening, you've at least got to ask yourself why she'd want to go - surely the only explanation would be to socialise or drink more. Unless of course she really just wanted to meet new people, and once that was off the cards decided that she wanted to sleep after-all. This is all providing the guys (or guy, let's be honest, she probably wasn't double teamed) didn't have an agenda - or chose not to act on it (because she would have told you, right?).
We texted/chatted all day yesterday. I had some major breakdowns throughout the day, excusing myself around company to go cry in the bathroom.
I don't know your wife, or your relationship, but reacting like that makes me think you're either in denial about something, or subconsciously know something might have happened. Your wife's story doesn't check out, and unless she usually texts you all day in situations like this then something is seriously amiss - either she's guilty or trying to patch something up which shouldn't need patching. The reaction you'd expect of an innocent person is pretty much 'yeah I was a bit stupid, drank a bit too much hun, see you later'. You 'okay, safe flight'. I'm also going to make the assumption (based on the flowers and getting dressed up line) that she got angry with you when you pushed her to tell you what happened which either adds to the insecurity comments above (something is causing you not to trust her), or means she was doing the classic backed into a corner thing I witness / experienced first hand, where a person in the wrong comes out swinging when they're backed into a corner - the guilt flip.
What likely went down
You're getting distracted by these outlandish worst case scenarios which sound absurd and very improbable and ignoring the obvious. She wanted to hang with the guys (or guy, see below). She wanted to drink more. She probably did both. She also probably knew that there wouldn't be a party.
You should ask yourself why she's lying. And in all honesty I think that's what's sent the alarm bells ringing - you know that she's lying but can't figure out why. The most obvious lie is that she went to theirs and stopped drinking and passed out. Why would she lie about what happened there? Well the threesome thing is hopefully (for your sake) ridiculous. But there's a far simpler explanation: she was interested in one of the guys to some level which made her want to keep the night going with him. She probably stayed up having a DMC with him, or shagged him, or kissed him. Or did something she doesn't want to tell you about.
Either way, you can't honestly think that what you've been told is honest. The story's full of inconsistencies.