Grettings, fellow Skyrimmers! I have an unanswered question that has prevented me from using manipulation spells the entirety of my tenure playing Skyrim...are manipulation spells effective late game? By manipulation spells I mean stuff like Furt, Calm, etc., which all seem to have a level cap fir their effectiveness.

Ohhh yeah. Very effective. Sometimes even too effective.

Illusion dual casting is worth getting here.

Depending on which perks you get, almost anything can be affected by Illusion spells. With Master of the Mind, really only dragons are immune to illusion spells. O, and guards, no matter what illusion spell you use, they're immune (at least as far as I know.) They fall under 5 categories
1. Spells that make sneaking easier (muffle, invisibility)
2. Spells that make a target hostile to all nearby targets ( Fury, Frenzy) They're good for assassinations.
3. Spells that calm targets ( calm) good for making targets leave you alone, like a saber cat
4. Spell that make targets run away ( Fear) sounds dumb, but its kind of useful early game
5. Spells that make followers more tanky (Courage) useful to followers who need more health and stamina

With dual cast and Necromage will I be able to use them on anything beyond bandits at high level? My fear is if I get upwards of 50, especially with the level cap removed, my character and playstyle will be rendered ineffective.

yes, you're spot on. Necromage raises the cap on high level illusion spells to 50. If you have master of the minds, even dragon priests are affected (they're lvl 50)

City guards will be immune once you reach level 40. There are a number of Master vampire's and mages who will also be immune.

This link tells the real story behind the effectiveness of Illusion spells. Scroll down some for a great table.

http://www.uesp.net/wiki/Skyrim_talk:Illusion/Archive_1#.22Higher_Level_Opponents.22

One can break level 51 effectiveness and even further with Necromage.

Illusion is effective but it's an all or nothing school.

I believe that guard can be manipulated if you get enough perks and effects that raise the level cap