Re:
Cassette bolt
Stick allenkey in a vice/workbench gap/imagination to hold it
use whole wheel if still there to turn it.
(for freehub from hub)
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similar for cassette locking (disc holding cassette onto freehub)
both should be steel for m900 unless the locking has been changed for a light weight. I guess not as it would probably have snapped, cut off by now.
Tool in vice, clamp etc.
OR clamp the cassette and use a decking long wrench on the tool.
If the tool doesn't have a location rod but a hole, bolt iit through.
If it is knackered anyway, buy a new freehub body. you don't need the XTR one, better would be a new Compact style one so 11T fit a bit easier.
Any pictures of the cassette ?
All else fails, swear a lot, hit the locking with a big hammer and come back at a later date.