Mr L,
At the risk of a little thread drift, I fear you may still not quite have a grasp on this VNAV thing...
In VNAV speed, you'll get an idle thrust decent at whatever speed is programmed in the FMC, so you can programme a 250kts speed restriction at FL100, for example, and the aircraft will decelerate to meet it.
In VNAV path, an optimum path, that is, line in space (but definitely not a speed) is generated within the FMC. This path will change with CI ansd weight, but not 'constantly' unless you 'constantly' change the CI on the Perf Init page. The path's validity as the most economic way of getting back to terra firma also depends upon the accuracy of the forecast winds input to the FMC.
So, in path, the aircraft follows the line in space. THERE IS NO OVER-SPEED PROTECTION. If the path has been calculated to be steeper than the aircraft can manage at idle thrust, the speed will increase, right up until you go through the barber's pole. (You will get a message asking for drag). If the speed decreases significantly (I think I recall it's 20kts below path speed), thrust will be added. Very commonly, this has to do with encountering positive wind shear shortly after commencing descent.
The Mark 1 Human Brain has phenomenal processing capacity; the 737 FMC is a bit of an Amstrad amongst Dells these days. I've given up on VNAV, and now only ever descend in LVLCH or V/S, and I'm very happy with that!