1) When using ALBs instead of ArcGIS Web Adaptors, the web context becomes "arcgis" and you would end up with the URLs below:
There is not a way to leverage ALBs but have the context be /portal, /hosting, etc. In order to choose the web context, you would need to leverage ArcGIS Web Adaptors.
2) ALBs don't support X-Forwarded-Host (or custom headers in the first place), which is why the URLs above have to have to context of /arcgis, instead of /portal, /server, etc. (corrected answer)