Proof that an XT "9 speed" rear mech works with an 8 speed cassette is available by clicking on the link in my signature to pics of the Max. I now have an XT 8s cassette before anyone mentions the temporary non-OCP one in the pics!
Cable stretch would be about right for 100 miles after a new cable is fitted. Rear mech alignment issue would have to be caused by something (crash, drop, being caught on something...) if it was fine before. In general, unless you have an off, any shifting problems that occur after a period of normal behaviour can be attributed to cable stretch or wear of components (cassette teeth, chain, etc.)
Hope this helps.
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