Complete novice gardener seeks advice on tomatoes and capsicums

I am in Perth, Western Australia. I have never grown any plants in my life, but this spring I bought some tomato and capsicum seedlings and am growing them in large pots.
I used no-name brand potting mix and fertilised with pellets designed for tomatoes. The tomatoes are staked.
I have a few questions:
1) At the base of the tomato plants, the brown potting mix seems to have been replaced with fine white sand. Is this a problem? If so, what should I do about it?
2) The capsicum leaves are being eaten by something. I have never been able to actually see pests, but small round holes appear in the leaves which gradually grow. Some leaves are almost completely eaten. The plants are just beginning to fruit (tiny at the moment) and I'm worried the bugs will attack the fruit. I'd rather use some kind of "organic" solution than chemical pesticide if possible.
3) Where the leaves join on to the main stem of the capsicum plants, the joint is black coloured. Is that just the way the capsicums grow?
4) The tomatoes seem to be going strong, but some of the leaves have small light yellowish-brown spots. Is this anything to worry about? If so what should I do?
All help greatly appreciated.

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