I agree with everything you say about the cloak, though I don't think we need to double-up on the insanity bit (ie, making wearer insane and part of it only functions for an insane user). Let's just make it a cursed item that makes the wearer insane.
Here's a rough draft:
Cloak of Figments: The cloak of figments is a cursed minor artifact that grants the wearer spell resistance 5? and immunity to mind-affecting effects. The wearer may also summon astral figment constructs at will, up to a maximum of 25? HD in existence at one time. Unfortunately, the wearer becomes permanently insane as per the spell insanity (in campaigns using sanity rules, treat the character as having -10 sanity points).
Strong conjuration, enchantment, abjuration; CL 20th; weight 2 lbs.
Does it need anything else?
animate object said:You may animate one Small or smaller object or an equivalent number of larger objects per caster level. A Medium object counts as two Small or smaller objects, a Large object as four, a Huge object as eight, a Gargantuan object as sixteen, and a Colossal object as thirty-two.
I could go for SR 17 or anywhere in the 15-20 range, as you like. I'm also fine with the limits you suggest and your edits to the insanity part. The bit about removing the cloak is good. Let's just break it into two sentences at "unconscious."
I'd move the paragraph about the insanity to the end, but otherwise it looks fantastic!
Astral figments are semi-illusory constructs created by the combination of powerful magic and a lunatic mind.
Is that enough?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.