Solid Hardwood, Engineered and Laminate Flooring - Engineered over radiant

I purchase engineered hardwood flooring for the entire house and now I am getting ready to install it. It is NOT snap flooring. We had gypcrete poured over a year ago for our radiant heat.

The questions:
Do I wait until the humidity is higher to install? Currently it is around 25% and we just started using a humidifier.
Do I use masons sand to level the floor?
Should I nail to the "stringers" that we put in the gypcrete (we thought that we were going to be going with real hardwood), or do i float the flooring?

Any other thoughts?

Over Gypcrete, your only opinion is a floating installation, as glue will not bond very long to gypcrete.

When the gypcrete was poured, it is suppose to self level itself, If you have high's and lows, something got away from them, and it would be in there best interest to bring the flatness up to industry and manufacturers standards.

Thanks!
The people who poured the gypcrete told me that they got it as level as possible, but there is supposedly some inconsistency since we had put sleepers in.
Their suggestion was to put in 3/8" osb over the gypcrete and float the floor.
Does that sound necessary?