Can I dig approx 17 feet and start layering my runs on top of each other, cris-crossing at 5 foot intervals vice running seperate trenches 5 feet apart horizonally? I just tonight have a golden opportunity to go geothermal when I was about to sign for a conventional air source heat pump. Also I am now in a time crunch for slab pour and need advice on laying for floor heat.

I have seen this done before. You should probably increase your pipe footage by 20% - 30% as a fudge factor.