What's your question? Ask

You can cut down the flower stem as far as you like, but make sure to leave the foliage as the plant needs this to store energy for next year's flower. You can spray the plants with neem oil, per directions, for pests.

If they're flowering, they may have reached or are near the end of their cycle. They could also be getting too much sunlight or have a watering issue. For additional reasons for browning in the leaves, this article should help: https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm

It depends on the cultivar because there are some dwarf varieties. However most varieties will grow to six or seven feet. Be careful because if you live in a fairly warm winter area they can become intrusive.

A lot of times, plants won't bloom the first year they're planted. It takes a lot of energy to put out a bloom and plants will most likely focus on reproducing themselves through the roots before they bloom. Insufficient sun and water, poor soil, and overcrowding can also prevent the Canna from blooming. One other possibility, if none of these fit your situation, is a lack of phosphorus, which is responsible for blooming. This can easily be remedied by adding a "bloom booster" fertilizer or some bone meal to the soil.

Cannas like it warm, so I would give them a little longer just to see if perhaps the weather (or soil) is not warm enough yet. If, however, they do not begin sprouting within the next couple weeks, it is probably safe to say that they are goners, likely to rodents.

You can cut down the flower stem as far as you like, but make sure to leave the foliage as the plant needs these to store energy for next year's flower.