Ok. Most of it will start making more sense when you've seen it in action! So obviously the VNAV path is largely theoretical, but if you keep VNAV engaged it'll follow it in VNAV PTH (subject to the limitations you've read about) and you only need to worry about speed. If you speed intervene in VNAV then it'll do exactly what FLCH would do: control through speed at the expense of the path. You would then control the path with throttle and speedbrake.
so to answer your question: yes you can control the speed in VNAV: open the speed window. You'll decouple (Start to deviate from the path). If you speed up you'll probably get low, so add thrust. If you slow down you'll probably get high, so use speedbrake. Now all you need to do is reset the FMC descent speed, wait 5 - 500 seconds for it to recalculate a new path, then use thrust or brake to get back to the path. Once you're close enough you close the speed window and it goes back to business as usual.