Is dual casting expert level Illusion spells absolutely necessary at higher levels to affect everything,is dual casting worth the perk point ? (character at level 81)

No. It's incredibly unnecessary. Especially with a character at level 81. There are some enemies that won't respond to certain Illusion spells no matter what; for example, I'm pretty sure you can't Pacify a Dragonpriest or a dragon. But all others, yes, provided you have the Master of the Mind perk (the one that allows you to control undead and automatons), and it doesn't take dual casting to do so. Once you get into Master level Illusion spells, the game is essentially over. But those are all dual cast spells so in terms of lowering magicka cost for those, it could be useful.

Not everything. But many things. Pacify works for Level 38 humans single cast. Which means it works on Bandit Marauders and Forsworn Ravagers. You will have to dual cast to affect Arch Necromancers and such.

nvm