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Yes, the phillips head screws on the periphery (4 in total) are no problem and the backplate sets in place uniformly all around. The three internal adjusting hex crews are used to set the position of the mirror, and there is a phillips screw for final set. But the hex screws are not really adjusting screws like on the secondary. They are simply designed to hold the bevel behind the mirror in place once it is set in its natural position i.e. by gravity as you describe. There is what appears to be a cork base which the mirror bevel settles to.
When I last used the scope which was many years ago, the natural set of the mirror base as you describe was perfectly aligned. Over time with rough handling it appears that this alignment has been compromised.
The problem is that if you use these three screws to attempt to adjust the position of the mirror when tightening the bevel simply reverts back to its natural position.
I understand that in later Mewlons Takahashi incorporated proper adjusting screws for the primary.
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