Have I derated my cable correctly?
So, 35 amps x .91 = 31.85 amps (albeit 14 guage must be restricted to 15 amp circuit breaker). I would assume based on this, that it would be safe to install the 14/2 NMD90 as planned? 


You are correct, the ampacities of #14 NM for 15A circuits and #12 NM for 20A circuits are conservative.  Even after derating for ambient temperature, you wind up using the same cables.  For this same reason #14 and #12 NM are typically used in hot attics without derating.