What your recruiter told you about open positions may actually be true because they are about to enter the new fiscal year. I just recieved my commission for the ER in May and at that time, there were only 3 ER positions left. My recruiter just sent down 12 applications for the NTP program and they all go through Med/Surg (supposedly but I am sure there are exceptions). So, I am sure you are being given next fiscal year estimations.
As far as going to COT, I don't think you will have to unless you're considered to be out too long. I know another recruit from my area was going from ANG to AD and was a Captain, he would not go through COT, but I don't know if it would have been different for him because he was active in the gaurd and not in the IRR time frame.
As for going to critical care from med/surg, I have heard 1-2 years is average, but if you already have med/surg experience, that may not be your case.
For your question about losing rank, I am not certain how that would work for you. I am assuming that you have your BSN. When I was accepted, they credited my years of experience in nursing on a 2:1 basis, civilian to military, for the first 6 years and then one year every year thereafter. As long as you've been a practicing nurse since your discharge, you should be getting credited years and I would think that you wouldn't lose rank in the process. Nor do I think it would adversely affect your retirement calculator, or if it did, not by much. As long as you left in good standing, no demerits, anything like that.
The only thing I will warn you is get ready for a LOT of competition. Selection boards have been very picky from what I hear, and prior military experience that should put you guys at the top of the list hasn't been as big a factor (as illustrated by that Captain in the ANG from before: he was NOT chosen when he applied). But I would absolutely start the application process sooner rather than later. It took right about 2 years for me to get in.
Hope all this helps!