Road splits are every bit as useless for Rockies hitters as home splits are. Luckily many of the players you mention have played for other teams too, making it quite easy to disarm your claims that no Colorado hitter couldn't hit .300 elsewhere. 

Matt Holliday hit .319 with the Rockies from age 24-28. After leaving Colorado, age 29-34, he hit .313, .312, .296, .295, and .300.

Gallaraga had more .300 seasons away from CO than he did with the Rockies.

Morneau's 2 best years for average (and his 5 best years for SLG% and OPS) were with the Twins, not the Rockies.

Cuddyer did get a big late-career boost from Coors Field, but he's the only one on your list that actually supports your argument. It is very likely that if elite players such as Helton, Blackmon, and Gonzalez had switched teams in their primes, their splits would have normalized after leaving the Rockies and they would have hit just fine elsewhere.