Can a potted live Christmas tree be kept indoors until spring (i. e. allowed to break bud and begin growing)? I plan on keeping it well watered in a sunny spot.
It can be done, but it is very difficult to do. Pine trees do not do well indoors on a long-term basis. You could maybe get it to survive, but it will most likely never thrive indoors.
Is there anything I should add to the water to keep the Christmas tree fresher longer?
There are many additives on the market that are supposed to help keep the tree fresh. However research has shown that just plain water is really all that is needed. Before placing the tree in its stand you should cut about an inch off the trunk base and then place that fresh cut into the plain tap water in the stand. If you forget to water it and the water is completely gone, then you will need to make a fresh cut to keep the trees ability to take up water in effect. Once the base dries out, resins form that stop the uptake of water kind of like a wound healing over.
Please can you advise me when and how should I repot my tree. I am not sure how old it is. It's about 2-3 feet high.
Check the roots. If they seem cramped, you can repot the tree. Here is more information on repotting: https://www.gardeningknowhow.com/houseplants/hpgen/learn-more-about-repotting-houseplants.htm
Where should I plant my rooted Christmas tree? It is approximately 3-feet tall. How big will it grow each year? I only have a small garden and am wondering how much room it will need.
It depends on the variety. Do you know the variety? If you let us know that, we can give you a better idea of how big and fast it will grow.
I would like to repot my living Christmas tree. It is about 4 feet tall, and I have no idea what size pot to use or what type of compost/soil is needed.
You would want to use a pot that is at least 2" larger than the one it is in now.
This article will help you regarding soil: https://www.gardeningknowhow.com/special/containers/proper-soil-mix-for-evergreen-container-plants-and-trees.htm
After Christmas, before planting the Christmas tree out in the garden, we need to leave it in a sheltered place outside for a week. Do we need to water the tree?
I would water it at least once or twice during the week. Its water absorption should slow as it goes into dormancy, but it will still need some water.
I have had my tree since late December. The needles didn't drop off like most trees after about 2-2 1/2 weeks. This has been the most unusual experience that has turned into a project! We keep it watered, away from heat sources, good indirect light and ventilation. I see clear sap in a few places on the trees and on the buds growing at the ends of some of the branches. The branches are stiff but it still
seems to be living. Should we be misting the tree or thinking about a sad farewell? Oh yes, we took off the decorations off long ago, I never use lights on my trees, and it is still in the tree holder.
If it is a cut tree without a rootball, then it is essentially a giant cut flower. It is impressive that you have kept it alive this long but as it has no root system,it will eventually die. But there is no reason you can't keep it for fun until it does die.
If it has a root system, then now would be the time to think about acclimating it to the outdoors and finding a place to plant it outside. Keeping them indoors only gets them through a short period and they will not live long term indoors. These articles will help you with acclimation and transplanting:
https://www.gardeningknowhow.com/houseplants/hpgen/move-houseplant-outside.htm
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm