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Kangaroo paws need little or no fertilizer, but you can improve their growth with a slow-release 12-4-8 fertilizer spread over the soil around the plant at a rate of 1 tablespoon per 1 square foot. Apply the fertilizer once every three months or according to label instructions. Removing the fans of flowers helps stimulate growth because each fan of flowers only blooms once.

https://www.gardeningknowhow.com/houseplants/kangaroo-paws/kangaroo-paws-houseplant.htm

Kangaroo Paw are hardy in zones 10-11. Gardenias are hardy in zone 8-11.
The first question you need to answer is if you are in the correct growing zones.
Kangaroo will need a near full sun location and Gardenias will need a partial shade location.
These links will help you.

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm

https://www.gardeningknowhow.com/houseplants/kangaroo-paws/kangaroo-paws-houseplant.htm

You should dead head the flowers as soon as they start to lose there color and nice appearance. Cut them down to the base of the plant, this will actually help the plant produce more flowers.

Although these plants live only in zones 9 and above, it is possible to keep the plants indoors. They do require a bit of a dormancy in which the humidity needs to be VERY low. Some leaf dropping indoors will be normal.

This article will help you to better understand the care requirements of this plant: https://www.gardeningknowhow.com/houseplants/kangaroo-paws/kangaroo-paws-houseplant.htm

Sure! This article will give you the information you need: https://www.gardeningknowhow.com/houseplants/kangaroo-paws/kangaroo-paws-houseplant.htm

This infection is, likely, in the soil. The best way to fix this would be an application of dolomitic lime and wettable sulfur. This will kill the infection, as well as recondition the soil back to a more fertile condition for plants, rather than fungal/bacterial microbes.