I believe much of your questions were answered on the AA thread.
You will not be able to use a cell while going through INS or Customs.

The trolleys from baggage pickup through US customs are free.

Are you coming in on AA or AR am a bit confused.
For you sake I hope its AA because it going to be a whole lot easier/quicker.

If AR unless your bags can be checked through to AA and your final destination you will have to take them down to the AA main check in are near concourse E/D to be checked, tagged and then to be handed over to security.

That can be a bit time consuming in that airport.
If such an arrangement does exist then there is an AA drop off just outside customs exit door, which is what you will do for AA anyway since they will be already tagged you just drop them off.

The walk in MIA can be a bit daunting for people unfamilar with airport.
I have timed it from E9 to A12 and it took 14 mins but that was inside security. The majority of AA's domestic flights leave from either A or D.

But you will need to do that outside security and depending on how busy the airport is just negotiating around the people will take longer.

Good luck.

mike