One of the Fallen Protectors is occasionally ran out and tanked elsewhere for awhile on many occasions during the encounter. Not to mention how He Softfoot loves to shadowstep all over the place. Don't just depend on Multi-Shot for your multi-dotting. Sometimes you have to throw out individual SS and I highly recommend creating macros for it(an example would be /target He Softfoot /cast Serpent Sting /targetlasttarget) I'm mentioning this fight in particular but this is just one good example. Making sure to have Serpent Sting on all targets that will live for at least a couple ticks is crucial for maximizing SV damage otherwise you are better off going BM.
I've found Glaive Toss to only trump Barrage on Immerseus(Barrage is better on Heroic Imm), Iron Juggernaut, Malkorok(I prefer Barrage on heroic Malk for MDing the adds), Thok, and Siegecrafter. Pretty much every fight I did not mention Barrage is better than Glaive Toss by a considerable margin, especially when timed/aimed/used properly.
Examples of good timing; It's worth delaying Barrage for a few seconds if you know Rook or Sun are about to their Desperate Measures on the Fallen Protectors encounter because a bunch more adds join the fray. On Norushen Barrage should be used and aimed properly to hit every Essence of Corruption on screen for maximum benefit, don't use Barrage if there aren't any adds on screen. On Sha of Pride I often have delay Barrage to make sure I have it for when Self Reflection spawns all those adds. On Dark Shamans once the bosses hit 65% it's very important you only use Barrage when Foul Geyser spawns all dem slimes, Barrage times perfectly with each Foul Geyser. Barrage is great to hit all of Nazgrim's adds even when they are spread out across the screen, always have a Barrage for each time he summons more.
Last but not least on Garrosh later on in the fight I like to save Barrage for when the boss Mind Controls several people. Extremely effective burst that can hit all of the MC targets even when spread out a bit. I know that's a long shpiel but as you can see I love Barrage. One last thing to consider is when the adds are clumped it's better to get in an initial Multi-Shot before Barrage so that while you are channeling your Serpent Sting is beginning to start ticking.
Dire Beast pretty much sucks for SV in my personal opinion, especially once you have the 2 piece as you stated you do have. It adds to button bloat and doesn't contribute much to helping you AoE which is where SV shines. My rule of thumb is if I'm going to spec A Murder of Crows I'm using Fervor(which is almost never), otherwise it's Thrill of The Hunt. What TotH does for you that many don't consider is that it makes your rotation more error proof. Using an Arcane Shot at the wrong time and paying the full 30 focus for it can often delay using much more important abilities due to lack of Focus. There are some amazingly fantastic players out there that manage their focus to a T and never delay an Explosive Shot, but can you or I say we are capable of such perfection?
Aside from that TotH is a tremendous boon to your AoE, especially when the AoE has to be done consistently like on Galakras/Fallen Protectors.
One last thing I'll mention...don't use Noxxic. It's a decent resource for beginners but it is not what you want to follow when you are truly looking to maximize your raid dps. There are much better guides at places like Icy Veins or even right here on the MMO Hunter forums up at the sticky topics.