if none of those treatments are working....chances are that the rash is from candida and those treatments will only make it worse. stop all of them if there is any question....and try treating the candida. see if you respond.
i am recovering from candida and currently have a rash just like yours under my armpits....i have had it for two weeks and finally figured out that i need to leave it clean and dry. wash with dr. bonners tea tree soap and keep it dry. get it in the sun...the sun will heal and dry it up.
in the meantime, keep the sugars and carbs. out or to a minimum and start on something like primal defense. this may cause more rash at first as it will chase it out of your system.