The My Cloud will generally reject access to it’s HTML page(s) from any domain/IP address outside of the local IP address subnet. As such trying to use a single bay My Cloud to host a web page may run into the same problem when attempting to access it from outside the local network.
Of course if one is www.mydomain.com/ I assume one has properly configured the records to point the domain address to the router’s broadband IP address.
Officially the single bay My Cloud is not really designed to host one’s own web pages. If one has a v2.x single bay My Cloud they may be able to use the third party app hack to possibly run their own web pages if there is a third party app available to do so.
No idea. Use the forum search feature (magnifying glass icon upper right) if you haven’t done so already, there are a number of threads discussing the inability to access the My Cloud Dashboard from a remote location. Perhaps one of those discussions has additional insight on what config files to change that may open up the ability to use an HTML page from another subnet.