On this point, it depends on a couple of things. I played union and league growing up, and while the league ruck is similar to a union ruck, there are a couple of differences.
Firstly, aside from the markers, everyone else is "back 10 metres" (unless playing the warriors)... This means that if there's a good play the ball, or the markers aren't set, the dummy half can attempt to run or engage the markers before passing without destroying momentum. It's especially effective if the dummy half can engage the first A defender before passing. Leads to time and space.
Secondly, passing off the ground straight away can be really hard in league. In union, the defenders aren't meant to move until the halfback picks up the ball. In league the ruck has a couple more variables for the dummy half. As a ball runner (e.g. prop/middle forward) timing your run can be really hard. Why? Because you don't know if it's going to be a 3 second play the ball, a 4 second PTB or a 5 second PTB. This is why you (too often) see a dummy half doing a "Justin Marshall", e.g. taking 2-4 steps before passing. The other complicating factor for momentum is that if the pass from dummy half doesn't get quite wide enough, the runner will be tackled early by the marker(s) nullifying metreage, the d line doesn't need to move back and slowing down the play the ball.
I really like your post from a union perspective and I agree with the points, just the league ruck is a bit different.
The league ruck sadly at the moment is about the wrestle and play the ball speed. Sometimes it's a better option to not try and fight the tackle for an extra metre trading off for a slower play the ball. It's really difficult to coach though - quite often it's experience that teaches this. This is why the warriors need to bring back more offloads and have support runners. Having 1-2 defenders having to hold wider means more yardage, faster play the balls and more time and space for the hooker and halves to do their thing.
Hope this makes sense.
*Edit*. P.S. Really agree with your last point that a lot of passing in both codes in NZ is terrible. So often it's the pass that puts someone into space or gives them the half step advantage to the outside which leads to a break. Accurate passing at speed is such a difficult skill.
Shaun Johnson is actually really good at it (from his touch background), but for most players it needs work and it's especially critical for halves, the fullback and the centres.