Apologies for the SUPER LONG DELAY to update on this issue.
The normal grievance procedure was not exercised in this case, and I was very suddenly granted the pay rate I was promised in my enrolment letter in summer of 2018, roughly 2.5 years after the grievance was filed. I received no paperwork acknowledging the error other than my EMAA statement that included back pay for the 2.5+ years, and the correct pay rate moving forward.
My advice to those in the same situation (not being paid what their enrolment letter promises)
1. Keep you enrolment letter (without this piece of evidence, I feel I would have had to wait for the grievance to process to complete, and I would have not been successful).
2. Do your duties and carry on, every single unit I transferred to needed to be notified of the grievance, and several of those units responded with a LOT of pressure to drop the grievance. Calling me entitled, and getting a lot of resentment from Junior Officers in my CoC. I even made photocopies of my enrolment letter, and a sample pay statement to show any doubters, and as briefly as possible, and said to them "lets get back to work, and I thank you for your support".
One grizzled Lt(N) once told me, in an attempt to get me to drop the grievance, "you need to ask yourself what you can do for the Navy, not what the Navy can do for you". I replied, "I'm here to do my duties no questions asked, while patiently waiting for the Navy fulfill its contractual obligations Sir."
3. Prepare yourself for some tax implications. I had to get additional documentation indicating that this was "back-pay" and that the funds were to be taxed over 3 calendar years, as opposed to what happens by default, which is a huge tax hit all on the year the arrears are awarded.
Thanks again to all that provided insights and support. To those that were butt hurt because I didn't respond to DMs, nor did I provide my unit, trade, rank; next time just move on. My purpose of posting here was to provide an example to anyone in a similar situation. This is not entitlement, it was a violation of a contract, and should be followed up on respectfully and professionally though proper administrative channels. I did my duty for 2.5 years with no hesitation while Her Majesty was in violation of my contract. That's how you get through this.