Viper skill page is a complete mess.

[Shurrikhan]

ST Fangs, AoE Maws.

Open first combo with Death's Design Noxious Gnash (+dmg taken from you debuff) via Dread Fang for 20s, then pick either Jinpu Hunter (Damage) buff with flank finisher or Shifu Swiftskin (Haste) buff with rear finisher. Thereafter, alternate, but using Steel Fang over Dread Fang except as needed to maintain Death's Design Noxious Gnash.

GCD-CDs (Winder/Pit buttons) open into Void Reaping/Cross Reaping Hunter's/Swiftskin where Hunter is again flank and gives Jinpu and Snakeskin is again rear and gives Shifu, again to be alternated (opener, A B; opener B, A). They also apply Death's Design Noxious Gnash for 20s while having a 40s CD, so that further reduces the number of Dread Fangs you have to use, though wholly overcapping on the debuff costs only 60 potency anyways, a la Armor Crush.

So ultimately you just have to remember whether your spammable combo finisher was most recently Flank or Rear and whether your GCD-CD combo finisher was most recently Flank or Rear... and alternate.

Now, the debuff provided by Dread Fang - Noxious Gash - can be provided by another skill available to Viper - Dreadwinder. This skill holds 2 charges and refills every 40 seconds. Since each charge applies a 20s debuff, and we have 2 charges, we can achieve 100% uptime on the Noxious Gash debuff simply by using Dreadwinder ASAP when it becomes available. Also, this skill starts a separate combo chain which is far more powerful than our basic weapon combo anyway, so this leads to maximum damage potential.

No, you cannot. Charges do not cool simultaneously, so only the actual recharge time matters for something's relative uptime. Dreadwinder provides 20s duration on a 40s CD. Accordingly, it will take the place of every other would-be Dread Fang, not all of them.