That's me.
There I am, talking about secret rooms in Mist Top and how beautiful it looks - and not realsing before right this hour that there is a secret laboratory as well which so far I have completely overseen. The door is at an elevated position in the secret garden, and since the beauty of the garden made me keeping my sights glued to the ground, I so gar have overseen it, until just half an hour ago.
Here is hope too that the author will increase the functionality of the mod.
I try again with the Riverlodge now, I just like it by the looks. I will deactivate Riverside Extreme (or somethign like thatr) and the Hunter'S Cab as well, and hope that when Riverlodge is the only entranc ein that area, it then will work. If not - return to former status.
That must be one of the most annoying bugs in Skyrim. Bethesda really should fix it.
I also hope that any further update to Skyrim will not make some of these mods unplayable.