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I'm not sure about the exact prep for Indiana but I would think that a bed of mortar would be worse than a bed of sand since the mortar would crack during any severe freeze /thaw. But I'm not sure how a sand base would heave, buckle, twist, etc.. during your Winter.
Of course the ultimate would be a good compacted base, as you spoke of, and a solid rebar reinforced concrete ( depth to be determined by those more familiar with your climate, I would pour 5" - 6" for mine )Then a mortar base under the flagstone would allow you to grout the joints.
I'm trying to envision your mortar base on top of the #73 and it just doesn't seem like it would hold up for any amount of time.
But then again, I'm from the South and don't have to worry about it!!!!!
Hopefully some other replies will correct any erroneous information I may have given you but that's my thoughts anyways.
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