Electrical – Need help with wiring a light fixture

If I understand this situation correctly, both red cables are Positive and carry the current from switches. You may want to check both red cables if they really correspond to these 2 switches. If so, I guess You need to choose which circuit (1st red + N + E or 2nd red + N + E) You want to use, cuz I guess You will be using just 1 switch/dimmer (as Your 'new' light needs only 1 Positive cable).
I would check (for safety meanings) red cables (if both run current while switch/dimmer is on), put 1 red to 'A', 1 negative to 'N', 1 earth to 'E' and ISOLATE remaining red cable (using some isolation tape or else). If everything goes well, 1 switch will turn Your 'new' light up and remaining switch would be obsolete.
I'm not sure about all the fixing (if it needs to be done by qualified worker), but I did something like that several times myself.

Two part answer:

The North American wiring coloring standard is black for hot and white for neutral. But unless you've performed the wiring installation yourself, you can never assume black is hot. You should obtain a house wiring tester and test which wire is hot and which is neutral. Consult your fan instructions and connect the hot wire to the hot wire and neutral wire to neutral wire (assuming end or parallel installation).

For the weight of the fan itself, it is best to determine if the ceiling box (if you have one) is attached to a structural member of the ceiling with a lag bolt. If it isn't, consider adding a new ceiling fan box that would attached to a structural member. You would have to patch some drywall but it's safer than having the fan fall down at an unexpected time.