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It it possible to rotate the mirror in its cell? That way you could confirm if the mirror is astigmatic or it's gravity tilting the mirror.
I'd still recommend putting a Cheshire eyepiece on it; see what the collimation is really like.
How much clearance is there is the cell? What about wrapping a thin strip of masking tape around the edge?
The mirror, like any optics don't like to be in direct contact with metal; there's a chipping hazard and expansion problems. The mirror should not be tightly held in place; just enough to "feel" it being able to move ( doesn't have to rattle, just "feel" loose).
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