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Potting soil will work. You can root them in water, but they have a higher success rate in vermiculite or potting soil.

As long it as looks healthy, you can go ahead and set it outdoors once weather permits. The following article should be of some help to you: https://www.gardeningknowhow.com/ornamental/flowers/geranium/how-to-save-geraniums-over-the-winter.htm

Yes, you can prune them now. Cut off about 1/3 to 1/2 of the woody stems on the plant to help get rid of the legginess and encourage new, green growth. They may not look great when you first put them back outside, but they recover amazingly fast. In just a week or two after putting them back outside, you will see them regrow and start looking great again.

Yes, these plants require full sun for optimal growth; otherwise, you may get leggy plants and poor blooming.

Yes, you can prune them now. Cut off about 1/3 to 1/2 of the woody stems on the plant to help get rid of the legginess and encourage new, green growth.

Yes, you just pinch them off. And by pinching them off, you encourage the plant to bloom more. This article will explain about it:
https://www.gardeningknowhow.com/ornamental/flowers/fgen/deadheading-flowers.htm