The only complete definitive information on a PLM would have to come from the Smarthome/Smartlabs Developer subscription. Smarthome considers that information proprietary. I don’t have a subscription so I cannot say for sure all you want to know about would be available. Generally we learn from our own experience or that of others.
Normally Access Points can communicate with each other between floors. Range on battery devices like the motion sensor or thermostat would be less, perhaps requiring more than two Access Points to cover dead areas but I would have thought Access Points between themselves would be okay. However, there are no absolutes in the business. If something is interfering with the RF signals being generated by the Access Points, more could be needed. I would try different plug locations before buying more Access Points. Mine are in opposite ends of the same floor but are communicating through a few walls. The distance between them would be less if installed on different floors. If the LED on the second access point indicated it was installed on the opposite 120v leg to the first installed Access Point I would think that was enough for cross phase coupling. More could be needed if wireless battery powered RF devices such as the Smarthome motion sensor had problems reaching one of the Access Points.
I would look at where the PLM is receiving power to start with. A simple test can be done by plugging the PLM into a good 3 wire extension cord and plugging the other end into a different circuit and different room away from where the PC/UPS is currently plugged in. This technique has worked well for several people in determining that the PC/UPS was attenuating the powerline signal, indicating a FilterLinc is necessary. If the computers are installed near each other, the larger FilterLinc can handle 10 amps so perhaps one or two is all that is needed. Of course if the computers are located in physically different locations more could be needed. Unplugging devices is also a good way to determine if they are causing a problem. Generally the extension cord technique is needed for the primary computer/UPS as they are needed to test with.
Insteon devices turning On/Off by themself is very unusual unless they have been assigned an X10 address. Then noise can easily turn an X10 address On/Off. There have been reported incidents where Insteon devices have been received from the manufacturer with X10 addresses already assigned and even active link records. Several folks on the Smarthome forum strongly recommend doing a factory reset on every new device before it is placed in service.
PC and UPS power supplies rank high in the signal attenuation category. I have PC/UPSs installed in different locations in the house and have put a FilterLincs at each location. Some TVs (not all for sure) have been a source of problems. I have a flat screen LCD in the bedroom and found it was causing a small problem. Found that one by pulling the plug. Reliability was not bad when it was plugged in, just got a little better when unplugged so I put a FilterLinc on it. Have not heard anyone talk about a refrigerator being the source of a problem. Hair dryer could be considering the type of motor but how often or long would that device be running.