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Nitrogen promotes green leafy growth in all plants. In excess, this growth will be at the expense of flowers. Obviously, plants need some nitrogen; it is the one major nutrient that passes through soil the quickest. Also, a fertilizer with lots of phosphorus and potassium can cause problems. A soil test is the best way to determine the nutrient needs of the soil in your garden. Without one, you are traveling blindly and at the mercy of fertilizer adds/product labels. Good cultural practices such as topdressing with 2 inches of compost, may be all that is necessary in healthy soil that isn't sandy.

If you fed a slow release, or anything high in nitrogen, then this will definitely keep them from flowering. They will need a food higher in potassium and phosphorus to do their best.

This article will help you to care for these: https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

Go ahead and plant them out as usual. The will be fine, just a little eager for warm weather, like the rest of use cold climate gardeners.
Shelter them just a bit, just like you are hardening off a new plant from the garden center--to help them adjust to the sun.

https://www.gardeningknowhow.com/ornamental/bulbs/calla-lily/growing-calla-lilies-and-care-of-calla-lilies.htm

Yellow leaves usually indicate too much or too little water. Check the soil moisture level by sticking your finger into the soil. Soil should be moist but never soggy. Make sure the water is draining from the pot, check the drainage hole to make sure it isn't blocked.

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

I found calla lily on the deer resistant list for several states but it was omitted on others. Check with your local "county extension service" for lists in your area. Tender, new, water-filled leaves (as opposed to lavender leaves) are what they favor along with flowers in the bud like tulips and roses. They do not like to come close to the house unless they are spurred on by hunger and they do not like herbs or fragrant leaves/flowers. What smells good to us is stinky to them. Consider interplanting sage, lavender, thyme or other aromatic species around and among the calla.