What advice can you offer to someone looking to transfer from AG to Army Nurse (TXARNG)?

You wouldn't be able to transfer to the Nurse Corps without a nursing degree in hand. If you're looking to transition into the healthcare side of the Army look into 70B which is under the Med Service Corps. From what I understand it's one of the few entry level positions in AMEDD for non-providers. They attend the same phase 1 of BOLC (although they have a different phase 2/track phase). This might make the transition easier if you do end up transferring to the NC down the line.

Well, investigations are done by CID, which is mostly a warrant field. Branch transfer usually happens at CCC, which means you're a few years away from being able to do so. Why did your supervisor recommend you move to another career field?

I don't know what to tell you about your current Guard career path, but I'll recommend a school to you if you are interested in Fire Investigations. Try Eastern Kentucky University, they offer that program either resident or online and my firefighter buddies say it's top notch. The school also does a 4 year program for EMT's. http://fireandsafety.eku.edu/
If you are looking to get the Army to pay for some training, I would look at 65D as a PA. A PA would be more inline with the emergency medical side of the world anyway.