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Although some resources do mention this, they do not explain why...and most resources for growing Lisianthus flowers say they are fine mixed together, though in containers you may want to stick with the same type as far as height and such. In fact, the dwarf types would be ideal for pots.

Thank you for bringing this to our attention. Our editors have made appropriate changes to make this less confusing.

Place Lisianthus 6-8" apart in well-drained soil with plenty of sun.

This article provides some additional tips for growing Lisianthus:

https://www.gardeningknowhow.com/ornamental/flowers/lisianthus/growing-lisianthus-flowers.htm

Yes, you can plant them out in your garden once you are past all chance of frost.

Lisianthus are grown as a perennial in gardening zones 7-11, and as an annual everywhere else, since the winter will kill them in colder zones. Even in zones 7 and 8, the plant requires protection during the winter (being cut back and having straw placed over it), since it is sensitive to even mild winters.

Yes, it is possible to collect seeds from Lisianthus. This article explains how:
http://www.dontveter.com/howtogrow/eustgran.html

In colder areas, another possibility is to dig the plants up and keep them as houseplants during the winter.

This depends on where you are located. In warmer climates that don't experience frost, they will be perennial. In cooler climates, depending on how cold it gets, they will either be annuals or biennials.

This article will give you more information on their care: https://www.gardeningknowhow.com/ornamental/flowers/lisianthus/growing-lisianthus-flowers.htm

We don't usually recommend places to purchase. I would recommend contacting your local nurseries, or many online nurseries will carry them as well. Add the words "for sale" to your search, and it will usually bring up results that are for sale.

Well, it isn't quite hardy in your area. It may be too injured to recover, but it never hurts to give it a chance.

You will have to grow these as a potted plant in your area, and bring it indoors during winter. This article will give you more information: https://www.gardeningknowhow.com/ornamental/shrubs/potato-bush/potato-bush-information.htm