Miami airport help!

Dear all knowing v-flyers!
MrB and I are flying into MIA on Saturday and then hopping down to Key West with AA. We have a tight turnaround (about 2.5 hours) so to maximise our chances, can anyone help with the following...
We will arrive at Concourse E and then have to find our way to the the North Terminal for AA - I think. Can anyone help with exact directions.
I assume it is a good idea to check in before leaving London, then we should be able to use bag drop? If we check in and then don't make the flight, does that cause problems later?
Sorry for so many dumb questions
Thanks
MrsB
MrB and I are flying into MIA on Saturday and then hopping down to Key West with AA. We have a tight turnaround (about 2.5 hours) so to maximise our chances, can anyone help with the following...
We will arrive at Concourse E and then have to find our way to the the North Terminal for AA - I think. Can anyone help with exact directions.
I assume it is a good idea to check in before leaving London, then we should be able to use bag drop? If we check in and then don't make the flight, does that cause problems later?
Sorry for so many dumb questions

Thanks
MrsB