I think the "leverage" one might have comes from rank and how long one has "been in"
Are you a recent graduate from a nursing program or do you have a few years as a registered professional nurse under your belt? The reason I ask is...if you have a few years under your belt sometimes no matter what you place on your wish list, the Needs of the Army come first....And you may end up at a post you never even thought of!
You will find out via Branch if one of your picks came through, or at least that was how it was for me....Some people find out via their orders....
Now looking over my USAREC Form 1205, I see that for the Generic Course Selection Program - GCSP, it states "I understand that if I request and am assigned to Alaska, Korea, or Germany that my selection of the GCSP becomes null and void" So the answer to one of your questions is YES, if you are assigned overseas, this will affect your critical care course guarantee. In fact, when you do go over the USAREC Form 1205 your Recruiter should be sitting right there with you to explain everything before you sign or initial on the dotted line.
I know of a CPT (AD) in the ANC who took and graduated from the 8A course (CCNC) as TDY Enroute to Germany, but as I said she graduated from the course first.
Whether you stay CONUS or attempt to PCS OCONUS is your decision.
<--- check out this link for more information on the different duty stations you might be assigned to.