Hello,

I am sorry about your gardenias.  Your pictures show a good bit of dieback. 
Dieback is a symptom of the plant dying usually seen starting from the tips to the center of the plant.  Some of this is a natural part of a plants life cycle but most often it is due to a disease, pest or environmental stresses.    My first guess from your information is that it is a stress problem.  I am having problems with my gardenias due to the excessive water and high heat.  

If you think otherwise, do you see any bugs on all the plants or more issues with the leaves than just a yellow discoloration?  Yellowing leaves is just an iron deficiency, as the nursery told you, which is caused by soil ph above 7.0, (maybe lime added to the lawn increase the ph).  To confirm, you can test your soil for free until November.  Here is the link for details;
https://wake.ces.ncsu.edu/extension-master-gardener-volunteers-of-wake-county/soil-testing-schedule/

I am adding a link to the Clemson University garden site that has pictures of common gardenia plant problems and instructions on how to care for the plant.  Look at this and see if this helps.  https://hgic.clemson.edu/factsheet/gardenia-diseases-other-problems/

Also, if you scratch the branch and it is green then it is still alive.  I would prune out all the dead branches and see if you get any new growth or if the problem persists.  Make sure that the shrubs have well drained soil as well. 

Let me know if you have any other questions.  

Thank you for contacting us and good luck!