It is a best guess, like ded-reckoning. You know the glide path angle (or it can be calculated from the altitude and the distance) and you know, or can estimate your groundspeed. Therefore you know how fast you must descend to maintain glidepath. You can check your work as you fly the approach using 300' per nm and can adjust if you are too high or too low. If understand if you are still skeptical, many are until they see it, but it does work. Most of the time though the range is pretty narrow 700-900fpm. if your approach speed is 140kts the descent rate will be about 800fpm and you can adjust as needed. If you error on the high side you can always fly a "dive and drive" profile but of the three airlines I have worked for that have taught and used this technique we were eventually prohibited from doing that. If we hit the DDA early we initiated a climb but not the missed approach. We waited until the MAP for that.

As far as the mixing you are correct in reference to the CRJ (ProLine 21?) the Q400 on the other hand (Thales with Universal FMS?) not only displays a VNAV path but you can also connect the autopilot to it through the flight director and if your airline is an approved operator you can use it if noted on the plate. So it depends on the equipment and the operator.