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You can simply remove the spent blooms, stem and all. In order for the plant to rebloom (container grown), it must go through dormancy. This article will help with that: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm

With seeds, you should at least begin to see growth within 3-4 weeks at the most.

Just prepare the planting hole around the same depth as the pot it is currently in and water it well. Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/flowers/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

Here is a link that should help.

https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/make-calla-lily-bloom.htm

Yes, dividing calla lilies is quite simple. Simply divide the rhizomes in the spring and replant/repot.

It could be a combination of shock (from the transplanting) and too much water, along with the fluctuating temps. Too much water can result in root rot, which would explain the yellowing and wilting. I would check the soil to see if the area is remaining relatively moist. If so, allow it to dry out some before watering again. Allowing the plant to dry some should help its roots bounce back. If this doesn't help and the plant still remains unhealthy, the roots have probably rotted and there's nothing you can do.