Will AirDrop use the data allowance from the phone and wifi modem

If you have a couple of computers kicking around to test AirDrop on, you can open a terminal on each machine and enter something like 'ifconfig -a' which will list all your networking interface cards. I'm not near a Mac or Unix box to show, but what you will see is a network interface that might be called say "en0" (Ethernet0) which is a physycal interface (may be wifi, or cabled) and a further one called "en0:0" which is a logical interface that piggybacks on top of the physycal interface. In this way you can run multiple virtual interfaces using just one physical one, and this is how AirDrop works. It is possible to manually create these interfaces yourself, and "multihome" your mac by having one interface say on a 192.168 address, and the other on say a 172.16 address, thus allowing it to talk to 2 networks at a time. It was always relatively straightforward on cabled interfaces, but I am not sure how tricky it would be (from a hardware support viewpoint) with wireless connections, but it's clearly possible in some way. See this question and answer for more details, but in short you need to be familiar with such terminal commands as ifconfig, netstat, route etc. Use the Airdrop network to access a computer?

AirDrop is very hit and miss. Sometimes, it can work consistently for a day, other times, it won't work for a week. I have issues with it all the time. I'm hoping this gets resolved in iOS 10 and OS X 10.12 this fall.