Hi Sam,
Unfortunately I can not see this being resolved in current Leia 18 OSMC builds for Vero 4K!
I just today upgraded via MyOSMC to 17.8-169 and when opening the Amazon VOD plugin (latest release 0.4.8 from GitHub - Sandmann79/xbmc: Prime Video Addon for Kodi Media Center) the inputstreamhelper popup “widevine drm library missing for this architecture” occurred with no download button available.
After applying the patch from the OP onto ~/.kodi/addons/script.module.inputstreamhelper/lib/inputstreamhelper.py the download button appeared and after installation the Amazon VOD streams are working again.
Btw, I already am on Leia quite some time via [TESTING] Kodi 18 (Leia) builds for Vero 2 & 4K.