Desolation, Rage of Rivendare

Hi, I'm not sure - those buffs affects disease's DoTs too? I know DnD deals 10% more damage if target has Blood Plague, but how about diseases? It increases Frost Fever's DoTs so that every tick deals 10% more damage, or only increases Icy Touch's single hit damage? Also Desolation - Blood Strike to proc it before applying diseases to have 5% more dmg - it means 5% more on DoTs also?
In guides I see it's good to Blood Strike first when 2+ targets to proc Desolation, but why not for single target then? Is it just not worth it?
And lastly, I just wonder why on Gnarly Guides they show Icy Touch as first in rotation, while Plague Strike should be first as it procs Rage of Rivendale?

Because Gnarly guides are most overrated guides to any class, their authors are far from great in playing what they are trying to explain and contain way more mistakes/missleading information than it should be.
Just ignore this trash of a site and check this forum guides cuz they were validated through years of practice and race with each other between players

It does. Best unholy opener is BS>PS>IT (specially if you run IT glyph). Don't guide yourself over things that were made 12 years ago by people that was completely fresh on the class and specs; you should always aim to watch (most, not all) Warmane forum guides that cover, at least, basic things such as a proper rotation.
One little tip on this specific topic> What most guides don't cover, for example, is that 1 BS is way less damage (always) than Blood Plague/Frost Fever 6-7 ticks + wandering plague ticks (up to a potential 12-14). You're sacrifising 25% less damage on a single BS to have not only desolation up but most likely trinket procs for your whole duration of Blood plague and Frost Fever which is a more significant damage buff for those diseases than Desolation and Rage of Rivendare themselves.