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I believe it would be best to leave it in the container for as long as possible. They don't like being handled, and may suffer quite a bit from disturbing its roots. This article will give you more information on their care: https://www.gardeningknowhow.com/houseplants/croton/care-croton-plants.htm

It is likely that they will, but you are right on the fine line on where they can survive. I believe that covering them will help. I would wait until the weather warms up to trim them, though. Here is an article that will give you more information: https://www.gardeningknowhow.com/houseplants/croton/growing-croton-outdoors.htm

Cut all dead leaves and branches back to the point of origin on the parent branch or to the main stem.
Cut back any long, overgrown stems just above a leaf set to maintain a compact, bushy shape.

Hard prune the croton, cutting the stems back severely if the plant becomes leggy with little leaf growth at the base of the stems. Remove only one-third of the total stem height at a time and allow the plant to sprout new growth before continuing to reduce the height.

https://www.gardeningknowhow.com/houseplants/croton/croton-dropping-leaves.htm

Sure, you could separate the multi stem plant. These are often planted in groupings for more color effect; so you can decide! Only use a pot that is 1 or 2 sizes larger than the nursery pot.
https://www.gardeningknowhow.com/houseplants/croton/care-croton-plants.htm

Croton (Codiaeum variegatum 'Mrs. Iceton').

There will be no such thing as too much sun for a Croton. In fact, the more light, the more colorful the plant becomes. What they don't like is cold temperatures. They will not survive a freeze. This article will help you to care for them: https://www.gardeningknowhow.com/houseplants/croton/growing-croton-outdoors.htm