You're saying one player accepted the charges as if Firmino had a choice in it. He was arrested for a DUI, which generally has conclusive, irrefutable evidence (breathalyzer or blood test). It's not as if he drove himself home buzzed/drunk and then turned himself in the next day because he felt compelled to do so. I'm glad that he seemingly learned a lesson from it and hasn't been a repeat offender to our knowledge, but I'm not going to pretend it was noble of him to accept the consequences of his actions when he was caught red handed.
I didn't compare the severity of the respective crimes/allegations. You said Klopp hadn't propped up his players like Arteta did with Partey, and I quite quickly found an example of him saying Firmino was undroppable 4 days after being arrested for DUI. I'm not keeping score, but rather pointing out that you're the one throwing stones in glass houses here.
I mean, yeah, it sucks that the reality is most of these cases don't have conclusive evidence and come down to a he said/she said debate. And that's true whether we're talking about Thomas Partey or some random college frat boy. The hard truth of it is that there has been enough public cases of false allegations against athletes, which unfortunately makes it harder for women who have been raped/assaulted to come forward. But I don't think it's any better to unfairly stack the deck against the accused and assume that everyone is guilty either.
As a fan, I'd rather not have a player even associated with such acts, but I can't fault the club for not suspending/releasing a player in the absence of clear evidence.
I think you have a very loose definition of "clear." The only supporting evidence (at least that I'm aware of) was those text messages / snapchat messages that the accuser leaked, which could very easily be faked or manipulated. I'm not claiming that they are indeed fake, but it's not as if law enforcement obtained those texts directly from a cell phone provider and we have certainty that they were sent by Partey. There is definitely a difference between that and the evidence available in the Greenwood case.
You can personally believe that he's guilty, but I don't think you have any better reason for thinking that than the people who believe he may be innocent.