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They can't really get rootbound in ground. This is just a signal that the plant is aged, and the ground has eroded. The best course of action here will be the mulch, lightly.

This article will help you to care for these shrubs: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm

It could be a number of things. There are many small black insects, so from this description and no way to see them, I can't really narrow it down.

This article will help you to craft your own pest control: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm

This is going to be a complex issue, but I have seen this often enough to know what the key problems are likely to be.

Being indoors, I'd have to assume that the temperatures are just about ideal year round. This is not likely the cause.

That leaves just a few culprits that are likely to be causing your issue...

The most common thing will be a pH issue, but this is offset by using the correct feed for these plants. Acid loving plants need acid loving plant fertilizers, which also corrects another common issue: Underfeeding. They are heavy feeders.

The last concern will be that they are not getting quite enough light. This is very tricky indoors. Most modern windows will filter out much of the UV light, so if your windows are more modern then you may consider putting it into more light during the day.

If this is not possible, then a horticultural light fixture will help. There are many to choose from and they each have their perks and drawbacks.

This article will help you to get these wonderful plants to bloom: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-care-to-get-a-gardenia-bush-to-bloom.htm

Sometimes this just happens. The best thing to do is what you have already done. Re-dip and hope for the best. You may have some success with letting the soil dry out slightly. Not bone dry, but enough to see that it needs a little water.
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/propagating-gardenias.htm

These plants do not tolerate change, at all. Their environmental needs are many, and humidity is usually the cause for bud drop. Really, almost any disturbance will stop flowering.

This article will go over how to prevent this, and what causes it.

The instructions for these types of fertilizers will vary and must be followed carefully to avoid injury to plant. To give your gardenia exactly what it needs, look for fertilizers that have been formulated for acid-loving container-grown plants.

https://www.gardeningknowhow.com/ornamental/flowers/gardenia/fertilizing-gardenias-in-your-garden.htm