Wow, you've obviously never flown a B737 in VNAV. You're saying that leaving it in VNAV at 230kts, the aircraft will not exceed 230kts? Yet you also say it selects 240kts to provide a margin for 250kt speed limit below 10000'.
Alas too many hours I have, my friend...
"**** in, **** out" comes to mind when you refer to VANV not maintaining 230kts in a DESC, as VNAV can only be as good as the data you put in the forecast pages for it to calculate a CDA. Incorrect winds are the most common problem when experiencing this, every second FO does that when starting on the line, not realising... An accurate forecast model allows for an accurate CDA with the VANV PTH speed as defined by CI.
In climbout, VNAV PTH acts similar to LVL CHG and pitches for the FMC commanded speed, that speed being defined by a virtual point 180nm after takeoff, based on the track direction at this position, the wind direction and speed as per TOC WIND, FL entered, ISA Dev and CI.