Gardenias flowers are white what makes them turn yellow
Gardenia tubifera is a variety that has color-changing flowers- they darken from white to yellow to almost orange as the flowers age. Most gardenia varieties have flowers that stay white, though. It depends on the plant's genetics.
How many crowns can/should I plant in a say 10" / 250mm pot?
Recommendation is planting crowns 12 to 18 inches apart.
https://www.gardeningknowhow.com/ornamental/flowers/gerbera-daisy/growing-gerbera-daisies.htm
I am bringing my gardenia tree out from winter storage. Is it OK to trim the brown leaves and branches? I have a lot of new green growth.
You can prune the dead or damaged branches at any time.
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm
my gardenias plant was doing well until it got cut down with the lawn mower.will it grow back,or not?
Oh no! That may be a tough one to come back from.
Watch for signs of growing back from the roots, that's the best hope I can offer!
Do you have any books on caring for gardenias?
We do not have books, but we do have plenty of articles for you to read. These can be found here: https://www.gardeningknowhow.com/ornamental/flowers/gardenia
There are tiny black crawling insects on our Gardenia flowers. Not on the plant itself or buds just on the open flowers. I have sprayed with NEEM and Sevin. The problem is that they are only on the flowers as they bloom. They may even be flying insects but they are almost microscopic and appear to be crawling on the flowers. These are mini Gardenias and each flowers has an abundance of these black bugs.
If you can include a photo, it may help me to identify the pest. This sounds like a few different pests. Most can be killed off with home-made pesticides, however.
This collection of articles will give you ideas on how to craft your own pesticides, along with many other articles that will help: https://www.gardeningknowhow.com/special/organic
Our beautiful Gardenia tree is struggling. It has developed some ugly large black "bunion" type growths on the joints of the larger branches. They are very hard and lumpy, range from 2-3 inches in diameter, and seem to have developed from the inside out. The bottom branches aren't blooming at all and the leaves are sparse and yellowing. The top of the tree is still trying to bloom and looks a bit healthier, but I am worried that the "infection" will ultimately take over the tree and kill it. What is this and what can be done?
This appears to be a canker and gall disease on your Gardenia.
These links have information and treatment methods.
https://www.freshfromflorida.com/content/download/11060/142651/pp54.pdf
http://orange.ifas.ufl.edu/cfnurseries/pdffiles/plantfacts/Gardenia.pdf