Rather odd, I just checked 32 epubs with HTML ToCs. When the hyperlink was correctly structured and styled, it worked though the popup ToC is easier since you can pop it up from anywhere. Testing hyperlinks to external sites such as when an author puts in a Bookbub link using a image worked to connect to the site. The limitation here for the eInk devices is that the web browser is pretty much an antique and many web pages don't even want to talk to it. Using the Kobo Desktop app or the iOS and Android apps does not have this issue since a more modern browser is used.
As for the font weight? My experience is that it works. There is the issue that some people seem hung up on in that the font used for the weight setting is not the current font but that is a cosmetic issue and does not affect the reading experience.
Code:
readingFontWeight\ChareInk6=0.61
readingFontWeight\IBMPlexSerif=0.71
readingFontWeight\Domitian=0.69