Ikkyo certainly is that one "illusive" technique. I believe a quote from O-Sensei goes something like, "Ten years to master irimi, and a lifetime for ikkyo..."
Over the years it seems that my teacher's ikkyo gets better! Doesn't it always seem to be that way? As it used to appear to me to be a technique about the elbow, I have more recently been noticing that it is more about immediately about taking center, and unbalancing uke's center (shizen-ni + kuzushi). Where as I used to manipulate the arm-->shoulder--->center to bring them to the ground, it now seems to be about turning uke's center back on him just long enough to rotate it (vertical rotation) back and (horizontal rotation) around my body (center point in the circle). Each of the three basic ways you have described so excellently can be used based upon the force, speed and direction of uke's attack. I can sum this all up by saying that once you use kokyu to gain control of uke's center, anything is possible.
For the pin, we were always taught to use pins where both arms (nage) are free to deal with both further attacks by uke, and attacks from others in close proximity.