Point of order, if you're aiming for competitive SM you should probably consider making some soup. That having been said, since I'm not sure what kind of lists you're aiming this at we'll just stay mono-codex and consider this list in isolation.
Anyway. Let's see... I think you're aiming for a Guilliman style gunline deathball? Calgar's a good replacement, good counter-charge and hits like a truck. However, while your Space Marines have anti-infantry covered (and then some), I think you're low on anti-armour. Probably very much so, depending on your local meta. Some more long-range punch would be a good thing as well, at the moment you can be outranged and thus, forced to move.
In short, ask yourself what your plan is when your opponent brings some tanks and/or Imperial Knights
On an aside given that your usual army is Nids, consider whether you'll like this playstyle. Barely mobile gunlines are not for everyone
Some more comments on list units:
-Inceptors actually make pretty terrible Ultramarines rules-wise given that they have Fly. There's little synergy there.
-I don't think you're going to be seeing a lot of Fire Storm from the Aggressors if your opponents know what they're doing.
And to answer actual questions:
How does Operative Requisition Sanctioned work? Do I take 1 CP away and 85 pts than I can add a single assassin?
You leave 85 points open as Reserve Points, and can then spend the 1 CP during Deployment
- which means you can send in the Assassin you think most useful against your opponent, giving you some versatility in your list... assuming you have all 4 models available
Would adding an assassin drop battle forge?
Nope. The army is battle-forged and the Assassin shows up later. Note that if your Warlord has the Imperium keyword you also have the option of bring 3+ Assassins as a separate "Officio Assassinorum" Vanguard Detachment without an HQ. Won't give you a CP though, and doesn't give you the versatility of the Stratagem, but at least it doesn't COST any CP either and it allows for a larger number of assassins.