Users reported I2P issues also in this forum thread:
I2P used to work in Whonix 15 now it doesn't in Whonix 16
Re-installation will most likely be futile, not helpful.
guy → GUI
No need. I2P in Whonix-Workstation worked earlier without that.
Very likely. Making https work on localhost isn’t easy.
This could be reseached as per https://www.whonix.org/wiki/Free_Support_Principle.