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It just depends on the cold hardiness of the variety of pansy you plant, really. Some varieties do better with the cold than others.

Another possibility is that you have microclimates in your yard that protect some pansies more than others from the cold.

It really depends on how far down they ate them and what your weather is like (colder weather will mean slower growth).

I personally would go ahead and replace them, just to make sure I get blooms in time.

The pansies might be ok, and the snap dragons, though less so. Both are likely to suffer some damage. The dianthus will likely not survive though without some really good protection.

Water them well. If you have twinkle lights, weave them among the pots. Or you can fill jugs with hot water and set them among the plants.

Then cover them all with cloth sheets.

This article may help:
https://www.gardeningknowhow.com/plant-problems/environmental/protect-plants-in-freeze.htm

Yes, it is ok to fertilize these plants during warm spells this time of year. In fact, most pansies can be fertilized regularly throughout the season with something specifically formulated for pansies, but basically any commercial fertilizer will work just as well.

The distance depends on the kind of plant, and the look you want. If you're starting from seed, the height and width of mature plants should be printed on the seed package; size information should also be on the tags of plants sold rooted. If you want open area around the plants, plant them wider than the projected width, e.g. 8" width could be planted 10", 12" even 15" apart. If you want a solid swath of plant, set them a little closer than the projected width, e.g. 8"width could be planted 8" - 6" apart. This article has some more information: https://www.gardeningknowhow.com/ornamental/flowers/fgen/mass-planting-ideas.htm