Core for core, power usage WILL be lower. As a package it will depend on how many cores are present, the clock speed and what the rated TDP will be.
But the discussion is about being able to upgrade to Zen2 - power requirements being one of the probable issues.
I think most people hope that they'll be able to upgrade to more cores - not the same number of them, just using 20% less power.
It will be for all AMD Zen 2 CPU's, if anything it might not be fore the APU's at all, APU's most likely come later
For such an AMD fan, you're quite badly informed.
Currently available BIOS upgrades are for 3000-series APUs, which will be based on Zen+. So basically, it's the same tech these motherboards support already - they just need additional data to detect them.
3000-series APUs will likely be launched in May.
Ryzen 3000-series based on Zen 2 (chiplets, 7nm) is coming later and compatibility with existing motherboards is being investigated by MSI (and surely by other manufacturers as well). Whatever that means. Maybe AMD simply didn't allow them to go public yet.
If older motherboards (chipsets) simply can't work with chiplets, then it's a sad but fairly controllable case - people will have to replace them.
If it's an issue with power supply or something like that, MSI will have to cover the more power hungry 12/16-core CPUs somehow (lock cores, underclock etc).