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It looks better with the sucker removed, good structure for a young tree.
Not all insects that use a plant as habitat and lay eggs turn out to be plant pests that feed on the tree and harm it. And one individual and one egg deposit is not a lot to worry about. As a preventive pest management technique, you can wipe it off manually with a cloth or cotton glove.

The yellowing and blotchy pattern of discoloration on some leaves is indicative of a nutrient deficiency.
Use a complete organic fertilizer now, then spring and fall, formulated for citrus plants, like this:
https://www.downtoearthfertilizer.com/products/blended_fertilizer/citrus-mix-6-3-3/

You are correct. They will die outdoors in the winter in your area. Fortunately, it isn't impossible to grow these indoors.

About 200 watts of light will be needed for these, indoors. There are many ways to go about it, but that will be the light requirement.

Outside of that, container care will be relatively easy. This article will help: https://www.gardeningknowhow.com/edible/fruits/kumquat/growing-kumquat-trees-in-pots.htm

Usually, I will recommend potassium silicate for strengthening stems, but in this case it could raise the pH to very unsafe levels for the tree. Without knowing your current soil pH, and being very precise with how much that you use in order not to fluctuate the pH, it would risk the health of the tree. So I can recommend CAREFULLY pinching the stem until you hear a light crack, and if it is too thick to do so, then lightly bend it back and forth up and down the entire height of the tree. This will lightly injure the plant to form a protective layer, but again, can be risky. Please include more information about the tree, and its environment, and I will be more than glad to help you.

There can be several reasons for leaf drop, but this is likely do to a change in environment. Make sure you are gradually making the transition to full sun. Under a shade cloth for the hotest part of the day.

https://www.gardeningknowhow.com/edible/fruits/citrus/leaves-falling-off-citrus-tree.htm

Those large white spots on the stem are Scale pest.

Remove the dead leaves from around the plant and discard.

Here are links to help you.
https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

It is difficult to get a close look at the stems but I do believe I see Scale pests on the plant.
Remove all the dropped and dead leaves and discard them.
Treat with Neem Oil; every 3 days for 2 weeks.

https://www.gardeningknowhow.com/edible/fruits/citrus/controlling-citrus-scale.htm
Keep the soil moist but not soggy. Make sure the container has a good drainage hole.
Your plant is quite stressed, if you can get the scale under control it may survive.