First of all, welcome to the X-plane community if you finally decide to step forward.
I am pretty sure all these questions have already been answered here and on a variaty of forums. You will find a lot of answers on X-plane.org too, or even just on google.
You will rarely be disappointed in terms of compatibility with Mac. Some addons are only developped for PCs, but as I say, it doesn't happen very often. I can't even think about any example right now.
Between Steam and the Laminar Research installer, I would definitely go for the installer :
-You will give a greater contribution to the developpers (I have read it here before)
-You will have an earlier access to beta and final versions
-You will have a better access to X-plane files and your understanding of the structure of the sim will be easier.
Some will be, some won't. The default aircraft have been improved. There has been a huge amount of work done on that, but you will not have the advanced functionalities of the FMS and other more complex avionic systems.
PMDG will develop more and more for X-plane.
Some other companies (Jar Design, FlightFactor, Simionic, Carenado...) have already been doing a fabulous payware job for X-plane. And this wil take you further into complex systems like FMS etc...
Expect to be disappointed at first with FMS, TCAS RA, weather radar... but all this is being improved step by step.
You will also be able to find really good freeware aircraft like the x737. In general, X-plane.org is a really good start if you want to get an idea of what is done in terms of addons.
I've seen a lot of posts about joystick issues. Mine (Proflight rudder and saitek X-55) have always been working fine with X-plane 11 and 10, so I haven't followed that much. You should look into it but you should also note that X-plane customer service is very very good. They fix bugs at a very impressive pace, and they provide a very good documentation for every "level of use".
You could have a look at that link for instance :
You might want to take a look at that to juge by yourself :
This one is in French but shows interesting stuff :
And if you want to see what a good payware can do :
I hope this answers your questions.
All the best to you,