If I can add to some of what you wrote:
Yes, accounting systems are extremely expensive, and they don't just control slot machines, they also can tie into hotel rooms, privileged parking areas, free slot play, etc. They are far more than just capturing meters. The hardware is proprietary for a reason: It is in essence a secure banking system. Can't have anybody hacking into one and turning themselves into a high-stakes player overnight and enjoying all the freebees.
You're partially correct as far as SAS. But SAS only communicates with the machine. The BV only communicates with the slot machine, and the machine passes on that info to the system when appropriate. For bills, it's just a meter increment. For tickets, it sends the ticket info to a separate TITO server via the system. Depending on what validation method is used, you could print a ticket without a server. Our casino uses system validation (as opposed to secure enhanced, for example), so ticket creation and redemption requires communication with the TITO server. If communication were interrupted, the machine would go into an attendant hand pay when cashed out.
BV's from the WBA series onward can independently recognize a ticket or a bill internally. There is no "fail a bill, check a ticket" process. Other than that, you're correct.
Mastercomms, NT setups, sentinels, etc all communicate with the slot machine via serial comms. Some are straight up RS-232 (WMS is a good example) and others are proprietary (SPC board anyone?). But otherwise they are all serial comms. Most of the newer games are trying to go with a standardaized communication protocol (RS-232) since that would be a cost saving measure. Just not aristocrat. Dunno why they gotta be special. Anyway, the player tracking as you say sends meter data and ticket requests, but also machine tilts (this includes door open signals), jackpots, change lights, electronic fund transfers, and we can disable machines either via the player tracking at the game or via the system. There's more but you get the idea.
Mastercomm itself isn't a player tracking system, it's just the hardware that's installed in the game by the casino; they are used for Bally SDS (which is a system). NT setups use ACSC (also bally). Sentinels are Aristocrat's Oasis. I forget what IGT's setup is called (Advantage, I think?). I've used SDS and currently ACSC, and between the two I think I prefer SDS. ACSC is nice with all the marketing features and the iview displays, but each time Bally wants to do an upgrade to fix certain bugs, it ALWAYS adds different ones. SDS doesn't have the bells and whistles but it's far more stable.
As far as the RS-485 protocol, I've never taken the time to look that up for ACSC. Our physical setup uses what we call GameNets. Each one supports a certain number of slot machines via rack mounted boxes containing "Arctic" cards. These interface cards connect to the home runs that go to each bank via Cat6 cables. From there it goes to "harmonica" boards and cat5s to each player tracking setup inside the game. It's all serial (and our setup is older, so it's not high speed). The harmonica boards look like unpowered 5 port network hubs. In theory it works well enough, but get one cable with a drink spill in it and the entire bank will stop communicating.
Going back to the original topic:
IGT keeps the Ccoms locked up tight as a closely guarded secret. I've got quite a few driving progressives, but if I ever have an issue with one I have to call IGT, as they have the software, keys, and access codes to work on them. I really can't do much of anything with them except cycle power if they have a hiccup. Not to mention, it uses fiber optic to communicate with the machines on the bank. Those wires are a bit fragile. The Ccoms then tie into a cisco integrated services router, which communicates with IGT Vegas via a T-1 with a POTS backup. So, while it's possible someone could create a home-brew to replace all of that, I kinda doubt it's gonna happen due to the riot IGT would throw at someone if they find out it's been done.
Hope all that helps a bit!