> 3.Working emulation of the Aristocrat slot machines
Assuming you mean the MK5 games, most of the games already work, people just need to clear the memory and follow the setup instructions (or in the case of most of the US games, set appropriate values in the set chips - most people forget to set the lines so the games can't be cleared; a 90 or 200 credit max bet setting works with all games depending on whether they have 9 or 20 lines).
The games which don't work (all of five games) have no external system to communicate with, the rest are bad dumps, of which most of which have working parents or clones, or alternate versions under a different name.
The only thing stopping most games from being considered working by MAME standards is an emulation bug which causes them to hang after around 50 spins, or if you get a hand pay in the older US games. Some games would be considered working but with imperfect video/sound, but for now the entire driver is considered not working. Eventually, the ROMs may be updated to include some basic NVRAM images so that the average MAME person can play them as they would have in the venue - coin up and go (unfortunately, note acceptors are not emulated at all yet).
The venues which are retaining these 20-year-old games are only doing so because they haven't replaced them yet. Some games also haven't been re-released on later hardware so the older versions are sometimes retained for that reason, although in my experience, a gaming machine is replaced with a gaming machine regardless of what the previous one was.
MK6 games are just placeholders for now. The only thing which works is the POST, which runs far enough to verify whether the dumps are good or bad, and then the emulated speed drops to below 1% while my CPU jumps to 100% for ten minutes and I give up trying to run them (I don't think they boot to graphics anyway). The MK6 does not use the same video chipset as the Dreamcast/NAOMI (the MK6 uses the PMX1 aka Neon 250 rather than CLX2) so this could be on the sidelines for years.
As for machines running in venues, anything pre-HD is dead, including the original 4:3 Gen7 games (none of which have been sourced or preserved yet, but the hardware is a late P4-era Celeron-based Linux PC anyway which would be an absolute pain to even think of emulating inside MAME, unless you want to sit for half an hour going through the POST at < 1 frame per second only to watch it crash afterwards, if it even gets as far as passing all the tests).