Timothy Walters Kleinert
I'm a little confused by semi-compliant vs passive resistance. Specifically, I'm a little confused by what you mean by "uke initially establishing a connection (or not) with nage via his attack".
OK, Let's start with compliant. This is compliant training:
This position, which is very common in aikido, only happens because because uke connecting himself to nage's center and then "following" nage wherever he goes. In this kind of ukemi, uke's intent is to allow nage to throw him no matter what nage does. This is often what is considered good ukemi in aikido, but it has no martial validity at all.
Semi-compliant, as I have described it, just means that uke initially connects himself to nage's center as if he were to going to take compliant ukemi. While waiting for nage to move he will also keep pressing lightly into this connection so that nage can feel the direction and intent. However, once nage moves, uke does not make any attempt to follow, but rather simply maintains the same energy he already established. If nage uses this energy to take control of uke's body, uke doesn't fight that, but if not, uke stays put.
In this type of practice it is important to have no attachment to whether nage is successful or not, since it is not a competition. It is just uke modeling the energy of the attack so that nage can work on blending with it. This level of ukemi has a much greater degree of martial validity than the previous one, but it is still very much a training tool where uke is allowing nage to perform the technique under very artificial circumstances.
Passive resistance is the same as semi-compliant except uke does not initially give nage a compliant type of connection with his center. The difference is where the energy of the attack is directed. He would grab or strike in a way that does not connect and expose his center to nage's center and then maintain this same energy as in the previous level, allowing nage to throw him if nage is able to find uke's center and take control of his body, but staying put if not. Uke still does nothing to try to stop or fight nage's attempts to control him, but also he does not try to help nage control him as in compliant training. He just focuses on maintaining the energy of the attack.
Hope this helps. It is difficult to describe these things in words, which is why I tried to make it clear that this was not meant for instructional purposes. It's the type of thing where I could show you all five levels in person in a couple of minutes, and it would be much clearer exactly what I am talking about.