Confusion over Mature / Adult split...
My interpretation of the policy, in terms that might put it in a more relatable frame of reference, is if it's something you'd see on broadcast TV, it's for a General audience (mainstream network television, etc...) So, something like CSI would be riding the line, but it handles the material in such a way that it's not particularly offensive to anyone (or shouldn't be...I know from work experience that people will find the most ridiculously trivial things to be massively offended about, sometimes...)
If it's something you'd see on a cable channel...Breaking Bad, Sons of Anarchy, etc...it's Mature. It's a little harder-edged, a little more gritty...not necessarily more realistic, but, yeah...stuff that wouldn't pass network censors, but isn't necessarily 'graphic' or 'gratuitous' by most standards (because, like I said before, some people will go off about anything...)
If it's something on a channel you specifically had to subscribe to...Sopranos, Game of Thrones, Spartacus, etc...where they aren't pulling punches in either violence or sexual content (but stop short of outright hardcore porn), then it's Adult.
Now, granted, that is purely my own interpretation of the standard. One of the reasons they don't have hard and fast rules of what, exactly, is or is not permitted is because standards vary from region to region or even nation to nation. You might see something on broadcast TV in Europe that American censors would never allow (nudity, for instance, although that barrier has been hurdled a time or two in the States), or vice versa (when I was in Sweden, twenty-five years ago, people kept going on and on about how much violence was on American TV...they were absolutely aghast at something like Miami Vice). What passes for Mature in one person's mind may be Adult in another (example--I've been a costumer for a burlesque troupe for a decade, now, so it's no big deal to me...my mother, however, would have an absolute fit if she knew some of the situations I've been in because of my job). What might seem a perfectly logical restriction to one person could seem unreasonably draconian to someone else. So, they've given some general guidelines, and an explanation of what the 'spirit of the law' is, and left it to us to figure out where stuff is best sorted, with the caveat that they may step in and shut things down if someone errs a little too far on being permissive with their ratings/content.
So, my take has always been that if you're afraid your game may go a little too far at some point for a particular rating, go for the stronger rating. Nobody cares if a Mature game has a lot of General content...but the inverse is not true. And, as a player, be mindful of where that line should be drawn. I've gone back and rewritten large sections of posts in some of the Mature games I play in, because some of the details got definitely too graphic. But I enjoy those games, and I don't want my GMs to get in trouble for content going overboard, so I self-censor on their behalf.
Now, all that said...given that shows like Breaking Bad and Better Call Saul had no age restriction on them and weren't relegated to late-night, behind-a-subscription slot, I think the game you're describing could easily and successfully be done at a Mature rating. I also think there are enough examples of R-rated cinema with those kinds of themes that you could easily bump it up to Adult...and which one you choose will largely impact just how graphic the players themselves get with what they write (I was a lot more graphic about the results of gunshots in the Adult-rated Shadowrun game than in the Mature-rated sci-fi game I'm in, for instance.)