Two items, first all cement contractors I talked to said to staple it to the foam. 2" high density for the weight of the cement and anything your going to be driving on it ( I store my 40,000 lb rv in my shed). Pex always at the bottom of the pour no matter the thickness, heat rises and then makes the entire cement mass a heat sink. Second using O2 barrier pipe on pressurized systems is the standard for all HVAC systems regardless the heat source and the price difference is minimal. This only makes sense since you will be using air vents to get out the air in the system and you don't want additional air entering the system.
I purchased a pex stapler tool from Menards for about $100 and it was the best investment I made. One person lays the tubing in place and the next walks along stapling it down, it is designed to be used walking along. If you lived closer I would let you use mine for free.
I did a lot of research on all my in floor systems beforehand and this is what I was told by all the most reputable contractors I know.