Hi Venom! I'm in a sort of similar place to you, except I'm coming back into the hobby after a break instead of starting fresh. I'm still getting used to 7th ed but hopefully I can help.
Straight off the bat I'm afraid the Decurion Detachment and Orikan wont improve your Reanimation Protocols past a 4+ save. The last sentence of the second paragraph of the rules for Reanimation Protocols states that "the required dice roll can never be improved to be better than 4+" (
pg 112). That being said, its not entirely useless to combine Orikan with the Decurion because his unit will always have 4+ roll, even if hit by something with Instant Death (which subtracts 1 from your roll). Combine that with the his unit being allowed to re-roll failed saving throws of a 1, and you've got a unit that will not die. Plus Orikan is always fun when he goes super saiyan.
The Monolith is an awesome sight on the tabletop and I have always loved it as a core part of the Necrons... but its just not that great in 7th ed :( If you choose to Deep Strike it (which arguable the best way to use it because you can drop it behind you enemy and start dropping your guys out of the Eternity Gate) then you're going to run into a lot of problems just because of the size of the thing. It has such a big footprint on the table that you're going to have a hard time not scattering into terrain/other units/table edges and having to roll on the mishap table (apparently its only precious space marine drop pods that are allowed to negate this anymore, not that I'm bitter...). On top of that issue, the Particle Whip is an Ordance weapon so if you shoot that you have to Snap Shot with the four Gauss Flux Arcs which is also annoying. Its still a 14/14/14 tank bristling with guns that can drop down anywhere on the table and teleport your army to it, and it often has a dominating presence on the battlefield, but just be aware that it is clunky.
Personally I think you might be throwing too many points into shiny, expensive units (Monolith, Nightbringer, Doom Scythes) and not enough into your boots-on-the-ground (mainly the Immortals). If there was one major thing I would change to this list it would be to remove the Deathbringer Flight to free up space for more numbers. Doom Scythes are awesome on paper, but they're also really expensive for what they can bring to the table. If you take them out that frees up a lot of space in the army. You can replace them with a squad of 3 Heavy Destroyers in the Cult (and change that mixed squad to all Destroyers, thats an elite killer rather than a tank hunter in my opinion) to keep your anti tank up, and that gives you 170 points to bulk up the army with. Bump up your Immortals to 10 men (85 points) and that can be the unit Orikan tanks with, giving you a really solid unit to face threats with.
That gives you 85 points to play with. You could add another 5 man unit of Immortals, which wouldn't be a bad thing, but I think the army would benefit more if you spent them on upgrades for the Tomb Blades and Destroyer Lord. They are sort of tax-units for their formations but they can be amazingly useful. Give the Tomb Blades Shield Vanes to make them more survivable, and Nebuloscopes to help them kill things and you'll see much better results with them for just 12 points. For the Destroyer Lord I cannot recommend a Warscythe enough, it makes him a serious threat in combat with 3 S7 attacks at AP2, Armourbane, re-rolling 1's to hit and ALL failed to wound rolls. Use the remaining points to give him the Nightmare Shroud for a 2+ Armour Save, Fear, and a unique "shooting" attack and a Phylactery for It Will Not Die. Now your Destroyer Lord is durable beast in his own right that can weather a lot of punishment and jump from cover to cover as he hunts down lone tanks/small units/the stragglers your shooting couldn't kill. He can also sit with your Warrior/Immortals to grant them Preferred Enemy and make their shooting more effective, but just remember that he can't be teleported with the Monolith. Personally I think that this is a more effective use of 320 points than 2 Doom Scythes, but I won't deny that Doom Scythes are fun.
A lot of people I've talked too aren't fans of the C'Tan because their expensive and random, but I'm not that bothered by competitive play and from a fluff perspective I love them. The random shooting can be iffy but also makes the game funner in my opinion, and the model is kick-ass. That's a lot of ticks in my book. On top of that his Gaze of Death isn't a shooting attack so you can use it even if he is in combat
Well this turned into a longer reply than I intended... I can get carried away with Necrons :p Like I said I'm relatively new to 7th Ed so I might not have the knowledge of more experienced and/or competitive players. Necrons have been my jam since I was 11 though so I hope my opinion helps somehow, good luck with the army!