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If the plants have just gotten into your garden, I would remove that first pepper.

Depending on the variety and it's growing conditions etc., you may only get a few peppers per plant.

https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-peppers.htm

This sounds like Powdery Mildew, though without an image I can't be positive.

This article has more information.

https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm

This can depend on where the leaves are turning yellow, whether it be at the bottom or top of the plant. It will also have a lot to do with the soil that it is is, the temperatures that it is exposed to, whether it is in container or in ground, and many other factors. Can you include a photo, and a detailed description of where the yellowing is occurring, and what changed when you started to notice?

You can, but there will be very little nutrients in it. I would recommend adding fresh compost to this each time that you plant. You can compost your old compost until it is completely broken down over time, though.

This article will help you.
https://www.gardeningknowhow.com/edible/vegetables/tomato/early-blight-alternaria-tomato-leaf-spots-yellow-leaves.htm

This link will help you.

https://www.crfg.org/pubs/ff/tamarillo.html