My son is 15 months and has never had an adrenal crisis (and he has never even vomited) so I am not the best person to respond.  However, it sounds like you did all the right things under a tremendous amount of stress.  I don't think it is "absurd" to take him to the hospital every time he vomits.  Particularly when he vomits more than once and is lethargic.  Even your endo told you to go to the hospital immediately!  I am sure your family means well but they just don't understand the seriousness of CAH.  It is always better to be safe than sorry.  It is much better to go to the ER unnecessarily than to not go when it is necessary.  It sounds like you did great!!  I don't know if it is necessary to go to the ER every time you give an injection, but if I ever need to give my son an injection I am sure I will take him to the hospital just so I will know that he is okay.  Never second guess your instincts or worry about what others will think when it comes to your children's health.  I hope if I am ever in a similar situation I will respond as well as you did!   

I hope your son is feeling better!  God bless!