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Protea plants should be able to grow fine in pots, but they do have pretty strict soil and water requirements. The soil and the pot need to be extremely well draining, or the roots will rot. They also like heat and lots of direct sun. If you're keeping it indoors, that might be part of the problem.
You can read more about their specific growing conditions in this article: https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm

No need to question the soil content and needs! Have a soil test preformed to determine the best way to amend your soil. Your nearest County Extension Office can help you with a soil test for a small fee. This will take the 'guess' out of the process and will most likely save you lots of time and money in this garden project.

All of the organic materials that you mention are great amendments, but the true determination is what is going on in the soil that you all ready have.

Here are some links for you including a link to help you locate your nearest Extension Office.

https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm
https://www.gardeningknowhow.com/ornamental/flowers/leucospermum/growing-leucospermum-flowers.htm
https://www.gardeningknowhow.com/extension-search/

Yes, I do believe that this could be a feasible option. Any method that you can use to get the seeds to proper germination temperatures will suffice. It will not matter how it is done, only that it is done.

They aren't dead, but they do seem to be disturbed. This seems to happen in compacted soils, soils that remain too wet, or otherwise too dense.

I would cut any affected blooms, leaving the others. Treat with a mild fungicide, and do your best to ensure that it doesn't remain too wet for too long. (If possible.)

Here is an article that will help you with their care: https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm

Here is an article that will help you with fungicide use: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Sure! As long as you bring it in long before it gets below about 23 degrees or so. I would recommend doing so, even before it gets chilly.

This article will help you to know what kind of care they require:

https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm

I'm going to paste links to
two different articles that may help you pinpoint the problem(s):
https://www.gardeningknowhow.com/plant-problems/disease/root-rot-in-garden-plants.htm
https://www.gardeningknowhow.com/plant-problems/disease/leaf-browning-in-center.htm
If you've had an especially wet winter and/or spring, your plant may have some root rot.

Here's another article about what conditions are necessary for a healthy protea:
https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm

It would also help to spread some mulch around the plant. That will help cool the soil and prevent moisture loss.