Has the Army put out any official guidance regarding medic brassards other than what is in the 25 MAY 2017 edition of DAPam 670-1?

Ma'am, the "MEDIC" patches you are referring to constitute a "subdued brassard" according to DA PAM 670-1. This one is not authorized on the uniform. The only brassard that could be authorized for medical personnel to wear is the "non-subdued brassard" which is the white armband with the red Geneva Cross. I have not seen them used in garrison or field training at any point in my career, nor can I find a reference as to whether or not they should be used in that scenario. I do know that if there were ever a need (i.e. conventional warfare with an adversary adhering to the Geneva Conventions), those non-subdued Geneva Cross brassards would appear by the thousands. In the meantime, there are plenty of other ways to identify medics in garrison or a field environment. Here are some of the ways I and other medics I have worked with ensured people knew who we were (I apologize if they sound sarcastic, because they aren't):

1. We told them. (Line Medics are rarely shy and timid after they've learned their craft)
2. Usually found near a vehicle or building identified by a Geneva Cross.
3. Carried an Aid Bag with all manner of patches and medical equipment clearly visible.
4. Answered to a loud cry of "DOC!"
5. Usually wearing a field uniform that is stained pink from fake blood.

Long story short, Ma'am, I have yet to encounter a scenario when lack of identification has carried dire consequences.

Also, most young medics have a sort of "Hero Complex" that will likely give them the mindset that someone shouting "Doc" or "Medic" is some sort of "Bat Signal" and they'll rush in to save the day. We may be arrogant, overly confident, sarcastic, and think too highly of ourselves, but I can honestly say that all of us mean well.

No Ma'am, which in a way defines that they are not considered authorized uniform items. With the same set of updates that disallowed flight patches, anything not approved in AR-670-1/PAM 670-1 and authorized by the Institute of Heraldry is not authorized.
I would develop a local SOP if you're concerned, such as "Aid bags visible with road guard vest attached" or "Aid Bag worn, marked by white engineer tape" if people don't know who their medic is.

Vehicle marking is a different matter altogether, but there is a LOT that goes into proper use of a badged aircraft or vehicle marked as a MEDEVAC asset, air or ground.

This is a good question I’d like an answer to as well. I would suspect that with the nature of the conflicts we fight against opponents who don’t recognize the Geneva convention, we wouldn’t want to identify ourselves as medical personnel. However, if something happened with a near peer force like everyone is saying it will, then I would expect that there would be some thought as to whether we would be identified as medics on the battlefield. I also believe that they would be the white sleeve bands with a Red Cross as those would be more visible and universally known as opposed to a camouflaged patch with an English word on it.