Yet Another Hardiebacker Question

Howdy.
I'm setting Hardiebacker in a bathroom tonight. 1/4" square notch trowel, polymer modified thinset, set the backer board and then nail it down with 1 1/2" galvanized roofing nails. Questions:
1. How soon can I work over top of the backer board? I'll end up sitting on top of fresh work to put down new boards. Should I let it set up first or can I keep working? Time permitting, I'd like to go straight to laying out the tile as soon as the boards are in. Or should I wait until tomorrow?
2. None of the nail spots on the backer board line up with the floor joists. Is that normal? Should I ignore the pattern and nail into the joists?
(btw: New exterior grade 3/4" plywood, glued and screwed over existing 1/2" plywood. 2x8 joists 16" o.c., 1/4" hardiebacker, 1/4" 6x6 ceramic tile with flexbond thinset.. Hoping this is okay.)
Thanks!
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Done.
1. Went ahead and worked over top of the freshly mudded boards but didn't lay any tile.
2. Used the nail spots on the backerboard.
If the tile cracks then I'll come back and post. Should be fine - the floor feels *very* solid.
wrote:

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