First of all, great work!
In google-chrome-stable on Linux, I tried to highlight and then underline. It seems the underline erased the highlight. But when I load the pdf in paperpile, both were present (which is what I want).
In google-chrome-beta on Linux, the connection to google-drive does not work.
In both -stable and -beta, if a pdf has been viewed from paperpile, then annotated in metapdf, annotation will not show in paperpile. I was not able to find a refresh pdf function in paperpile.