1. It's about as reliable as your power supply and internet and local network connections. Where I've had trouble is my sometimes flakey local WiFi. So now I have it connected with an ethernet data cable. Don't skimp on the Pi power supply.
2. SA, at present (it may change in future), can only monitor one source of battery data, be that a BMS or a shunt or via an inverter. So if your battery's BMSs are connected to each other and one acts as the main comms connection to the SA Pi then yes it can report on each battery,
provided it is a compatible BMS/comms. But if you are hoping to connect each battery BMS individually to the SA Pi then at present SA cannot do that. Having SA monitor more than one source of battery data is an oft requested feature and is on the long list of features to add. There is no time frame for this feature.
3. I bought mine direct from SA, it was just easier as it was my first Pi and I wanted to shorten the learning curve. It was a Pi 3b. SA has guidance on hardware requirements here:
Flash SolarAssistant on to a Raspberry Pi in a few easy steps.
solar-assistant.io
I would stay with the officially supported models as listed. Note that the Orange Pi is not supported. That doesn't mean it won't work but that SA has stated it's not going to provide support for it.