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Particles shouldn't get loose on a hard finisher. The only crumbly stones I have are for sword polishing when a tiny sliver is used & is expected to break down.

As above, I use it as a guideline. More like 15 seconds than 15 minutes . Often just 5-6 X strokes works for me.

This does not sound good but if you have worked out your stones & get good shaves then you can be happy.

If I follow Iwasakis paper to the letter I get a perfect result tho, tomo nagura and all....

Both these questions could be qualified a little by what you call finishing.

If by finishing you mean 3-5 light strokes or similar then yes there are are really good hard & also slightly less hard...