Well most Insteon modules do no have both RF and power line signaling in them.
The new dual mesh lamp module is one of the first RF and Power Line modules they have released.
All of them do act as repeaters on the power line.
The Access Point module is a Dual Mesh device. It can repeat commands on the power line and send an RF signal to other Access Points. To bridge the phases and cover areas with poor power line signals.
Smarthome has come under much criticism for its description of all modules are dual mesh; in some of the Automation Forums.
In this case I believe the repeat of power line signal is what they are referring to for modules. RF to bridge phases and extend coverage to far reaching areas. Their white paper on Insteon seems to say it that way. RF by Access Point (or early SignaLinc RF) and repeated power line by modules.
Insteon modules link to each other. Some can act as a controller and responder. Some as a controller only or responder only. Technically you could link 417 devices to one button.
Yes a module can be linked to more than one button and each button could have a different mix of modules assigned to it.
One BIG point. Though any setup can be done with the mechanical set buttons on the modules. In complex setups it gets to be a BIG CHORE. Most then have to get a hardware interface and software to do it.
While X10 can sell you a complete AHP system with all the software for $49.99. My UDI ISY99i with Insteon PLM is around $349.00