Ok, you are quite right in that the knee near the spine is NOT necessary, I used that phrase so beginners with the position will try and focus on getting good knee and leg position. You are also right in that the position is mainly used for control and you can abandon it if you need to to attack. The important concept is that you have something pinning the shoulder, whether it's your knee or your thigh or even part of the frame. I like to think of it as trying to get your knee to the spine, but if that doesn't help you to think like that don't worry!
The important part is that you have control of the shoulder, if the opponent can free their shoulder, they can free their arm...
It's also worth noting that your knee should be close to your chest, but at the same time it should be covering the opponent's shoulder.
As for the omoplata, that becomes a bit more difficult without seeing it, so consider these more guidelines than concrete solutions. Firstly, is the opponent hugging your free leg? If the opponent can prevent you from getting the knee of your non hooking leg out it can much more difficult.
Also how far away is your head from your opponents? You can be on your side which is good, but if you aren't at a good body angle in relation to your opponent it can be difficult. Ideally you want to be at a perpendicular angle from your opponent, if you can see your opponent's ear that is generally a good sign. Here is a good example...
If you aren't at this angle, you want to use your free leg to push off the hip (again you need your knee free) to turn yourself
Here you see Melanson use the foot on hip to change the angle (though he goes to a high guard not an omoplata here) notice initially he is looking at his opponents forehead, before he changes the angle and can look at the ear.
Does this make sense? Hopefully it helps, let me know if you have any more questions or how it works out!