There has to be some way to access the pages from resources in other contexts!
Yes, thats true.
I am on the cloud, default install... and after switching yesterday the Systemsetting "Context Forwarding"
to yes i could access IDs ouf other ressources out of the box without any Problems at all.
Maybe setup an empty development instance and verify... it works out of the box.
I had some issues in the past at the cloud, and support fixed that very very greatly on the fly...
did you ask them if this could be an Technical / Server issue?
In my case Gatewaymanager (which is used to route Domains to Contexts) fired that "oops, not found Page of modx" no 404... but a issue slightly in that direction you have actually.