Q: Will this version allow for music input from an iPad/iPhone?

A: Yes, this does have cell/music input on it. 

Q: Does the inplayed music stop (or be muted?) when pilot/passengers start a conversation to other persons in the cockpit/plane? Or does the music stop (or be muted?) only by incoming or outgoing radiotraffic? I hope latter is the case - cause if not, enjoying music while flying seems quite difficult. Thanks in advance for your answer. :)

A: Any incoming audio from radio or conversation on the intercom will mute the music.

Q: We will be flying an aircraft with only a two place intercom system, there will be three of us aboard. How will this work in conjunction with the aircraft intercom to allow all three of us to communicate?

A: You will need to purchase the Double Headset Y Adapter to allow all three of you to communicate with a 2-place intercom system.  You can see that product here https://www.mypilotstore.com/mypilotstore/sep/1974.

Q: Is the PA-400ST capable of playing stereo sound from the music input or does stereo just mean it works with stereo headphones but only in a monaural mode?

A: Yes, it can play stereo music when connected to stereo headsets.

Q: Is there some kind of convention among aviation intercom manufacturers not to give the buyer a clue how long the cords/cables are? I would buy immediately if this info were available.

A: The cords from the intercom to the start of the plugs is five feet in length.

Q: Can a mixture of headsets be used?

A: Do you mean multiple brands? Sure, any fixed-wing general aviation headset will connect to this intercom.