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The most common components for Bonsai soil mixtures are Akadama, Pumice, Lava rock, organic potting compost and fine gravel (grit). You may want to purchase premixed Bonsai soil for your seedlings.

https://www.gardeningknowhow.com/houseplants/bonsai/first-bonsai.htm
https://www.gardeningknowhow.com/houseplants/bonsai/bonsai.htm

There are quite a few fungicides that you can go to, but I would tend to lean toward a product by Actinovate. It is a granular fungicide that I have used to treat many serious diseases.

If it is severe and persists, then you may have to turn towards copper fungicides, which can be a little more harsh on the soil.

This can be a difficult thing to predict, as this can be affected by whether it is in container, or outside, what its light conditions are, watering habits, and so on. They can be very tall in nature, but controlled in container.

This article will help on the care of these: https://www.gardeningknowhow.com/ornamental/trees/pine/loblolly-pine-tree-care.htm

Sure you can! Just keep in mind that this will completely change the shape of the tree, overall. If you do not mind this, then this is ,certainly, a viable option.

This article will give you more information: https://www.gardeningknowhow.com/ornamental/trees/pine/pine-tree-pruning.htm

Probably not. It isn't very strong, or intended for serious infections such as that. I would recommend wettable sulfur and dolomitic lime (going very easy on dolomitic lime) or copper based fungicides. They will be much stronger.

This will require a lot of patience. These can take months to germinate. I would leave them on, but I don't think it would harm them to take them off. Just whatever is easier for you. This article will guide you through the process of this: https://www.gardeningknowhow.com/ornamental/trees/pine/how-to-grow-a-pine-tree-from-seed.htm