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It could be a lack of nutrients in the soil, or that the plant is getting more sun than it did last year. Both of these can influence the vividness of color on the flower. It may also be that the original plant was grafted and the scion (the yellow blooms you saw last year) died and was replaced by the rootstock which produces a different color (pink).

To much fertilizer is likely cause.
To much Nitrogen will cause lush green plants and no blossoms.

You can add Phosphorous with Bone Meal fertilizer.

Here are a few links for you with more information.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm

There can be a few reason for buds dropping including insects and growing conditions.

Here is a link to help you pinpoint the cause.

https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-flowers-hibiscus-blossoms-falling-off-plant.htm

Recommended pH for growing hibiscus is about 6 to 7. These articles should help with correcting your soil pH: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/lower-acid-soil.htm

Yes, there are different types, namely hardy or tropical varieties. Your friend likely has a hardy type, while yours is tropical since it says to bring it inside. Hardy hibiscus plants need little care over winter and are hardy to zone 5 with no protection outdoors, though you may add mulch if you like. The tropical varieties are the ones that require growth indoors over winter.

If it is a hardy hibiscus, then it can stay outside to Zone 5. If it is a tropical hibiscus, it must be brought in for the winter. The tropical varieties must be grown in containers and overwintered indoors.These plants will not tolerate more than a night or two of light freezes. Even one hard freeze (below 25) could kill the plant. The hardy varieties don't need any special treatment, though wrapping or mulching of the plant in colder zones is fine. If you are unsure of what you have, I would grow it in a container indoors just to be safe.

Here is more info: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/wintering-hibiscus-indoors-winter-care-for-hibiscus.htm

You would not be able to put the plant into a Koi pond.
You can grow Swamp Hibiscus rain garden, poorly draining areas or on a ponds edge.
Here is a link with growing information.

https://www.gardeningknowhow.com/ornamental/water-plants/swamp-hibiscus/swamp-hibiscus-plant-info.htm