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Herbs are quite easy to grow, both indoors and out. These articles should help: https://www.gardeningknowhow.com/edible/herbs/hgen/growing-herbs-in-containers.htm, https://www.gardeningknowhow.com/edible/herbs/hgen/general-care-for-your-herb-garden.htm

Yes, indoors or out, herbs and many other plants can be successfully grown in compost only. In fact, plants grown in compost will typically be healthier than those grown in the soil due to the nutrients found in compost.

Yes, you can. Just make sure the pot is large enough to accommodate both root systems and ensure that they have the same or similar growing requirements.

No, you can plant them outside anytime you want. When cold weather comes they'll die back, but you'll have them till then. If you move them indoors before frost, put them in a sunny spot, and they may well keep growing over the winter. https://www.gardeningknowhow.com/edible/herbs/hgen/growing-herbs-indoors.htm

The humidity comes from the water evaporating from the dish. The pebbles are to hold the plant pot above the level of the water, so that the soil doesn't stay too wet. The herbs should be watered whenever the top of the soil is dry; the water should run down through the pot, out the drainage holes, and will add to the water in the tray. You can add more water to the tray as it evaporates. You'll be able to see the tray and pebbles, and add water as needed - probably once a day, or even more often.

Although some herbs are suitable for your climate, others require warm, dry climates and don't tolerate cold weather. However, many tender herbs can be moved indoors during the winter. You can also grow annual herbs, which are replaced every spring. There are so many choices!

This article is about evergreen, cold-tolerant herbs that would work in your climate: https://www.gardeningknowhow.com/edible/herbs/hgen/growing-evergreen-herbs.htm