What about platform = windows_x86 (since you’re on windows)… PlatformIO should then provide all the compiler toolchain itself… native is the odd one out - since it uses the ‘native’ toolchain that you’ve already installed (or so it assumes). 
And it is frustrating… I remember posting earlier about a way to increase the delay between uploading and serial monitor, which seems to be all that’s needed to close that last issue re unit testing. Can’t find that post again now… probably used an extra-script to introduce a brief delay after upload… thus no file modification needed.
EDIT: FOUND IT! 
SigFox MKR1200 Unit Testing - If you don't see any output for the first 10 secs, please reset board - #3 by isa56k