Yes, during our initial and re-currents it is explained as being due to fan (blade) separation damage control.
Our FA's and Pax regularly go into the hold at and above FL400. - Which is totally allowed.
We get a Blue CAS message, and if the door remains open for 5 minutes we get an Amber CAS message. Usually the Door is not open for that long. If we were restricted to not being able to use the hold door above FL400 we would seldom be able to use it, most of our trips over an hour are above FL400.
In answer to your other query, I am trying to find the document that has the diagram on, but I recall that the G cabin length is to the back of the aft lav, so not including the baggage compartment. BUT until I find it (or measure it myself!) I cant swear to it.