Alright, so...
1. Yes, you can mine anywhere. You get to planetary rings by going to the planet, then flying into the ring. When you get close enough to the ring, you'll enter it, just like you would drop out of supercruise anywhere else.
2. There are specific systems better than others. The quality of ores goes Icy (unmineable), Rocky, Metal Rich, Metallic. The quantity of ores goes No Reserves, Low Reserves, Average Reservers, Major Reservers, and Pristine Reserves.
You want to look for Metallic, Pristine Reserves for the best stuff. I am told the type of star affects it as well, but haven't done research into it myself. I can suggest a few good places if you want.
3. This depends on how good you are at scooping, how many lasers you have, and how lucky you are with finding the ores and percentages you want. At my best with two lasers, I made about 200k an hour. I did not have the best setup, nor am I particularly lucky. I am also kinda shaky with scooping. It does take a while though. Even with a hauler, it can take 2-4 hours to fill 16+ slots.
4. Trading is TECHNICALLY better, especially since the update, but mining is close, I'd say. I think early game (ships smaller than Type-6) mining is as good as if not better than trading. Once you reach 100+ cargo space though, I think the time involved in mining ends up making it a slower credit-maker than trading.
Oh, and planetary rings are just simply better than belt clusters. You can get up to a couple hundred in a single cluster. You can get tens of thousands in even very small rings.
I like to avoid resource extraction sites myself and just go straight into the ring at a random point. More lonesome, but less likelihood of pirates, in my experience.
Also, I recommend listening to Louis Armstrong and that type of music. ESPECIALLY "La Vi En Rose" it is perfect for mining.
