I'm still laughing at "security guarantees"...
In regards to the USN's Aegis warships (in the Med and Baltic), any missile launch by Russia that is determined to have a trajectory that would enter NATO airspace can be intercepted by them.
The issue of Russia launching a "tactical" nuke at the Ukraine would have to be a case by case situation. There is no doubt that the mobile launchers have been tracked and current positions known.
Also, considering the Ukraine's excellent track record of intercepting and destroying Russian missiles so far, I suspect that Russia would have a hard time "nuking" the Ukraine, since Russia wouls be launching a very limited number (one, two, perhaps?) as opposed to the many conventional missile fired so far - as mentioned upthread, out of 70 conventional missiles fired at the Ukraine, only 8 made it through. That's a hell of a fine job done by Ukrainian forces, to be honest.