1/ As Kalpak mentions, the 8051 sink the current, so when the port pin ==0, the led is on and the logic outpt of the opto is '0'.

2/If you need to use an inverter, it is probably best on the mpu side.

3/ Separating the boards probably won't do too much unless you take great care with the power supply and layout.


Using optos won't necesarily solve any problems. Careful circuit design and layout are required. Keeping the digital signals away from the analog section and proper power supply bypassing and maybe separate power rails for the analog section. There's plenty written in ap notes from Analog Devices and Linear to name a couple.