The keystone effect comes only from one problem: that projection screen and LCD panel inside the projector are not perfectly parallel.
If anything else caused it, such as some internal misalignment of the projector lens, which is extremely unlikely, you'd have a blurry picture.
Since that's not the case, the reason is very simple: You don't have your projector set up properly.
However you measure what is "level" does not matter one bit as it is NOT THE HOUSING of the projector that produces the image, but the LCD chip inside of it. You cannot possibly know its exact position and orientation to be able to claim anything. Even if Panasonic was to give us a clue as to exact location of the LCD panel, it would make very little sense as it would have to be in 3d, and that;s almost impossible. The only way would be to print the exact plane of the LCD panel on at least two sides of the projector, like on the top and on the side, and even then - how would one use that to measure how parallel are those small lines to a large screen?
Therefore, it is much easier to start from this simple fact: screen and LCD panel which are parallel, will always, with no exception, produce an image which has all sides parallel; EVEN when the lens is de-centered. With that in mind, one can also work backwards: if there is any keystone effect on the image, meaning the edges of the projected image are not parallel, there can only be one cause: the LCD panel is not parallel to the projection screen. So, work backwards: continue adjusting the position/orientation of the projector until you achieve perfect geometry of the picture.
Panasonic may have taken all this into account when they created the mount for the projector; if I were Panasonic, I too would not be interested in knowing what other mount someone may be using as there is no way to know how that mount would influence the geometry of the picture.
If you don't understand it all, just to illustrate, look through your window and notice how some cars or buildings or people seem smaller because they are further, and some appear larger because they are closer. Now, if you wanted to make sure that we all saw exactly the same thing, we would all have to be the same height, and in the same building, and looking through the same window... hence, Panasonic has all the right to ask their users to use only their mounts so that Panasonic knows all the variables. Otherwise, it is a shot in the dark.
As for me, I would just use any mount I like, but would take full responsibility for the quality of the image, and not expect the impossible: The keystone effect has nothing to do with one's level or any other measurement- it has everything to do with the orientation of the LCD panel in relationship to the screen. If those are not parallel, keystone will be present, a sign to continue working on the setup some more. Once the geometry of the image is good, you have achieved the correct position/orientation of the projector.
This is all nothing new, but since I have been working in the industry of digital images before most people ever even heard of such stuff, I feel a bit qualified to explain this (in truth, I really don't like it when a good product gets bad rap for someone's lack of understanding).
Just to illustrate how these things work: have you ever seen how a DP on a film set adjusts the focus? Do you think they look through the viewfinder and adjust it the way most photographers do? No, and the reason is that a movie is projected on a huge screen and even the minutest error will be blown up and easy to spot. So a good DP, in a close up shot where focus is critical, will NOT rely on focus ring and lens, but will rather measure with measuring tape the distance from the actors eyes, to the so called "film plane" which is always indicated on film cameras (and some pro still photo cameras, such as Nikon). Then, once that measurement is taken, they will dial it in on the lens, taking into account all other possible things such as variations that happen due to changing focal length on a zoom lens and so on.... I am only mentioning this to show that adjusting a lens/image plane with relationship to the subject/projection screen is no small matter. It is to be taken seriously, but luckily at least when it comes to setting up your PJ you only have to do it once. And then, make sure no one bumps into your projector.