1) Yes one ability is not that good, that doesn't mean I will replace it with a kick the fuck ass ability for you to further stack fear effects.
2) Yes Fear of the Mindless needs that cap to be less stupid than it already is.
3) indeed. You can try to find something else to slot in that spot.
4) Okay...
5) As a general rule, I am pro finding swift immediate actions, but a) your fault for choosing a class I didn't have a hand in making, b) I see you've already found several uses, c) my feats, as before, will work on these abilities as well, technically, though I'm sure they will be tremendously broken when applied to them, because anything Sinister makes is already 3/4ths broken even before using my feats.
d) Mentat Sense is fucking retarded. The reason Spheres don't give buffs is because if you have at will buffs, then they are active all the time. 60ft of always on best sensory perception in the game is already way better than what anyone else gets for so little investment, so you can just hold it to 60ft, instead of getting 100-200ft.
6) How good it is depends on what you are paying to get it, and how necessary it is to your character. And you know, it's made infinity times better by the sheer fact that Sinister allows you to break all immunities for no goddam reason. See below, 8.
7) Attempting to Overton window shift me by starting with a leadership feat, and then saying "But since that's too powerful, how about a further stacking fear bullshit." is not going to work. See 8.
8) So here's an average combat according to your current character design:
a) Enemy Appears!
b) Enemy is demoralized by your aura.
c) Enemy is Demoralized by your swift action.
d) Enemy is Demoralized by your move action.
e) Enemy is Demoralized by your Standard action.
f) Some of these add more than 1 point of fear, and add 1 point even on successful save.
g) If any enemy gets 2 points of fear, they immediately lose.
h) What if there are multiple enemies? Doesn't matter, half those effects are AoEs.
i) What if they are immune to mind effecting effects? Doesn't matter, all of those are ignored. Some of them are slightly harder to resist, some are not.
j) What if they are immune to fear? Doesn't matter, half your effects are demoralize effects, which now penetrate fear immunity. Also, many things are not explicitly immune to fear, because instead they are implicitly immune to fear by virtue of Mind affecting immunity.
Bottom line, I see no way to possibly challenge your character whatsoever, aside from using creatures that eternally stay outside the range of most or all of your effects. A quick SGT against the first 4 encounters I have results in you walking harmlessly through the world with things running in fear of you forever.
So for starters: 1) Don't take Master of Terror or Dreadful Demeanor at all, because skill feats in the Tomes are incredibly stupid and overpowered.
2) Fear of the Mindless only applies to abilities granted by the Nightmare Seeker class, not Demoralize effects you already had by virtue of skill ranks, or effects you gain through other means.
3) You can never use your move action to demoralize ever, because move actions are for moving, not for attacking.
4) There is probably some way, by feats or by race, or something, that you can get a fear aura, you can still do that, and if it takes multiple feats, we can stack them into one. But just a standard fear aura, and see Fear of the Mindless change.
5) Nightmare release can be switched out for a swift action Demoralize on a single target.
So instead a typical interaction with enemies would be:
1) Fear Aura (with a save), fear immune or mind affecting immune would negate this.
2) Swift action single target demoralize, Fear immunity would negate this, but not mind affecting.
3) Standard action whatever you want, but I assume mostly something that does some fear on a successful save, and more on a failed one. Back up non-fear stuff if you already found out they are fear immune.