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That is a bit cold for a lemon! If you can, bring the tree indoors for those temperatures. I would advise removing it from the clay soil, and getting a good citrus mix, or add sand to potting soil. This clay will restrict its ability to take up nutrients. There are many citrus fertilizers available, and some are meant to spray the leaves with. Here is a link that will help: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

They are pretty hardy from the wild. Be advised that they may, or may not be edible, being a wild fruit. Use your own judgement on that one. Here is an article that will help:https://www.gardeningknowhow.com/edible/fruits/lemons/transplanting-lemon-trees.htm

These can be a little tricky. From the looks of it, the tree lacks nitrogen. A high N (nitrogen) fertilizer will help bring back the color. Also dolomitic, when applied to the sol, will keep the pH from swinging on the alkaline side, which will also harm the tree. Make sure the trees receive plenty of water.

Here is an article for more information on caring for citrus: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

1. Lemons should do well in a tropical rainforest location. I couldn't find much information on growing lemons in truly tropical climates, but this article explains how to grow them in Florida (subtropical) but the information should be mostly the same:

http://edis.ifas.ufl.edu/hs402

2. and 3. Lemon seeds (either purchased or from a lemon you eat) can be planted in a small pot of soil. Keep the pot indoors and water it so that the soil never dries out until the seed germinates and the seedling begins to grow. When it gets bigger, you can plant it in a garden. This article gives more details:

https://www.gardeningknowhow.com/edible/fruits/lemons/propagating-lemon-seeds.htm

It should also be fertilized, but can do fine in gardens where there is high natural fertility. This article provides details:

http://hendry.ifas.ufl.edu/HCHortNews_CitrusFert.htm

4. Here is a list of tropical fruits that should do well in the tropical rainforest garden, and links to more articles about them:

https://www.gardeningknowhow.com/edible/fruits/fegen/growing-tropical-fruit-trees.htm
http://www.faculty.ucr.edu/~legneref/botany/trofruit.htm

Yes! These flowers are a sign of sexual maturity. They will start producing a few fruit within a year or two. It may not be much at first, but it will be something neat to look forward to. These will have to be brought indoors over winter in your area. They will not survive long below roughly 35 degrees. I would bring them in far before it gets that cold, though. They will need a south facing window to be happy.

Here is an article for more information: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm

Hopefully you are not using any pesticides on this lemon tree, or that may be the case. Bumble bees don't live very long, though, especially the worker bees which pollinate plants. It's most likely just their natural life cycle. This website may help
https://bumblebeeconservation.org/about-bees/faqs/finding-dead-bees/