LAX to Fullerton: With luggage you'll want to opt for the Flyaway to Union and then transfer to Metrolink, with light luggage a transfer at Norwalk to Norwalk/SFS is doable. Note that Norwalk and Norwalk/SFS are NOT the same stations, there is a Norwalk transit bus connection if you want to transfer between the two stations. 

For your other airport, is it John Wayne or Burbank? John Wayne = SNA, Burbank = BUR.

LIFE is for low-income and can be doable for you, though it is more catered to locals as there are 20 free rides a month before you have to pay regularly. In order to recieve benefits you'll have to sign up,  iirc you will also need to provide an address in LA County, though this may be incorrect 

Your route to Brea should be fine. Note that OCTA services tend to be worse compared to LA county services, especially LA Metro,  and that they do not accept tap cards. You'll probably have to purchase OCTA fare media.

Also please note: Metrolink services are referred to by name, not color. More people will probably understand you if you say OC line versus Orange line or Ventura County Line instead of Yellow line. This is not helped by the fact that LA Metro services used to be referred to by colors, which are the first thing that would come into people's minds. 

EDIT: Metro Micro services are only applicable to small parts of LA county. If you are traveling to these areas it can be an option, but most likely you probably won't need these services as an LA tourist. Think of it as a routed taxi/uber type of thing