The maximum number of devices you can connect to a 1-wire bus is depedant on several factors such as cable length and outside interference but there shouldn't be any problems with using 6 devices on a bus.
Maxim has a good tutorial on their website on building reliable 1-wire networks, you can find it at https://pdfserv.maximintegrated.com/en/an/AN148.pdf
You can connect the ADC Pi, IO Pi and 1-Wire Pi to the same Raspberry Pi. I would recommend stacking the 1-Wire Pi on top of the other two as the RJ12 socket on the 1-Wire Pi is the same height as the mounting kits so it can be a tight fit with other boards stacked on top.
You should be able to send the data from the inputs and outputs on a Raspberry Pi to Home Assistant on an NUC over the LAN. One method would be to use an MQTT broker such as Mosquitto on the NUC to act as a message store. The Raspberry Pi can then publish MQTT messages to the broker each time an input or output changes and Home Assistant can subscribe to the MQTT broker which would notify Home Assistant when a new message arrives.