Oh Man, Cedar's are screeming sirens for me. Luring me into their beauty only to die in conditions most tree's do fine in. I would have to say do your root work in the middle to late winter when it is still cold and then put in partial shade. I would never do any root trimming in the fall but that is just me. I'm on killing my second one right now but I think it will make it. It's one of the rare green(ish) Atlas cedars the last of Brent's collection. Short of going to the Atlas mountains of Morroco I do not think you will find any green ones around anymore in the US.
All I can do is share with you the kiss of death on these things and that is unsifted soil. If you don't sift your pumice and red lava rock and your source has allot of dust, they will get root rot and die. First the aphids and ants will invade and then you will realize why they are there.
"Age and treachery shall always overcome youth and skill"