While the degree of difference can vary depending on other factors, lacing radial spokes heads out can result in a small increase in stiffness over a crossed lacing pattern. Keith Bontrager tested otherwise identical wheels with either radial (heads-out) or 3x lacing, and found that the radially laced wheel was 5% stiffer laterally. There is little difference in bracing angle between radially heads out and 3x, so the difference in stiffness is mainly due to length of the spokes (3x spokes are about 5% longer than radial spokes).
As you note, radial heads-in lacing can result in an even greater difference in lateral stiffness, because this lacing pattern can result in a meaningful difference in bracing angle. In
Damon Rinardl stiffness's test of whee, he found that otherwise identical radially laced wheels, lacing heads in resulted in 13% more lateral stiffness than lacing heads-out.
Yes, obviously a longer spokes will have less longitudinal stiffness than shorter spokes. A 290 mm spoke is 7% longer than 270mm spoke, and will stretch 7% more under the same load. In turn, if the spokes stretch more, the wheel will deflect more. Similarly, a thinner spoke will have less longitidunal stiffness than thicker spokes, so thinner spokes make less stiff wheels.
If it isn't written about, its probably because for a given rim and hub combination, there is little difference in possible spoke lengths, so differences in spoke length will have result in only minor differences in the stiffness of that wheel. The number and thickness of the spokes and the bracing angle have much more variability, so variations in these will have much larger affects on stiffness.