Hello, first i must warn you, that my Ryzen 2600 arrives next week, so what i will tell you are just the fruit of googling in the past 2 days, before buying.
1-2) PBO can push above normal boost, but you need 2600X (or other "x" CPU):
Precision Boost Overdrive essentially combines the Precision Boost 2.0 and Extended Frequency Range 2.0 to deliver elevated performance as and when needed. You will need a Ryzen 'X' processor and a 400-series motherboard to leverage it. While the AMD B350 chipsets do auto-overclock 'X' CPUs to an extent, the B450 chipset's AMD PBO ability allows for much better performance gains when using a Ryzen 'X' processor.
3) I am not sure, but i have read many people saying that if you get an "X" version, it's better to just let it do its thing, between PBO and XFR. Hardcore overclockers might think otherwise.
4) From what i read, as soon as you change multi and voltage, all boards get with stuck frequency. Workarounds i 've seen: Overclocking through P-States (ASUS Zenstates users claim success) or overclocking through offset and using "weird" multi in Gigabyte boards.For example, instead of 3.8, make it so it does 3.75 with offset. Then it keeps the power savings with the lower power states. I have no idea if this works on MSI too.
Hopefully some more experienced user will give you better answers.