Does hideous laughter make a flying foe prone and cause the foe to fall?

A witch that’s using the flight hex to fly 20 ft. up in the air fails her saving throw against my halfling bard’s hideous laughter spell. The spell says that an affected creature

collapses into gales of manic laughter, falling prone. The subject can take no actions while laughing, but is not considered helpless.

Does the witch become prone in the air? Does the phrase take no action mean the witch falls to her death, grappled and subsequently slain by her own foolishly placed black tentacles?

Hideous Laughter while climbing

I found this question about hideous laughter being casted on someone who is flying. My question is for when it’s cast on someone when they‘re climbing.

The spell says fall prone, but it doesn’t say anything about them releasing their grip from what they’re holding (e.g. a ladder like what my PC in my campaign is climbing). Maybe I’m being pedantic but common sense would rule that, if they fall prone, they fall from whatever height they were at. No? Yes?

Is it possible to use warlock’s Hideous Blow in barbarian’s rage?

According to the Complete Arcane book, warlock invocations are not spells. Barbarian rage description in PHB1 says:

While raging, a barbarian cannot use any Charisma-, Dexterity-, or Intelligence-based skills (except for Balance, Escape Artist, Intimidate, and Ride), the Concentration skill, or any abilities that require patience or concentration, nor can he cast spells or activate magic items that require a command word, a spell trigger (such as a wand), or spell completion (such as a scroll) to function. He can use any feat he has except Combat Expertise, item creation feats, and metamagic feats.

So, if invocations are not spells – can I use it in rage? If they are spells can I just multiclass into Rage Mage from Complete Warrior handbook? In the other words invocations are arcane spells or not?

Hideous Laughter: What actions are available?

Ah Tasha’s Hideous Laughter, or simply Hideous Laughter in Pathfinder.

My monster’s fell fast and hard to this spell last night, and though it was a wonderful victory for the PCs, I’m now filled with trepidation at the apparent power of this 2nd level spell.

My first question revolves around the possibilities for the afflicted character/monster, the spell reads [emphasis my own]:

This spell afflicts the subject with uncontrollable laughter. It collapses into gales of manic laughter, falling prone. The subject can take no actions while laughing, but is not considered helpless.

Now, a helpless character means:

A helpless character is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy. A helpless target is treated as having a Dexterity of 0 (–5 modifier).

So, since they aren’t helpless and they can’t take any actions…what exactly can they do once they succumb to Hideous Laughter? — Can they move/stand up? Shift? Take attacks of opportunity? Defend themselves?