It's probably clear now, but in summary

• Initially, Aranwen was rejecting the connection on assumption that it was a trick of some sort. At this point, Saeriel had minimal control of Aranwen's left hand
• As Aranwen came to accept the connection with Saeriel's song being the main thing to change her mind, Saeriel could take more control of Aranwen's left arm, and Aranwen allowed it, encouraged it even
• In that moment when Saeriel borrowed Ch'dau's falcata to aid in the fight, she and Aranwen moved in unison, joined by blade and song
• Afterwards, Saeriel could not relinquish the control she had gained over the left side of Aranwen's body. She had become too embedded, so to speak

Hope that makes sense  

...especially once Ara stumbled over the steps in the turret room; which is a big part of the reason I had Ch'dau carry her out of the house when our little group was making their retreat.

That's a maniacal enchantment to put on those two.

It wasn't even the intent of the original enchantment, either. This effect only happened as a side effect of Kithran breaking Morgana's concentration, but the spell continuing to be effectively held by the affected, changing its effects in a way that wasn't supposed to happen. 

...that something as simple as a glancing blow from a dagger to the shoulder can affect the results of a spell like that, tbh.

Nice touch, Butterfly.

Though feeling a teensy bit guilty 

I don't think you need to feel guilty... Kith might've actually saved Ara (and Sae) with her intervention. Just from a dumb Kazari's POV, anyway.

Plus, Ch'dau actually got to meet Saeriel in a manner of speaking which I think is cool AF.

Oh, no worries, it was just a teensy bit, I've done worse things to your characters than accidentally altering a spell after all

And yeah, that meeting Sae aspect of it is really neat!

What would have happened if Morgana's actual enchantment had gone off?

Morgana was weaving several spells together in that moment, part of why she didn't bring much magic to bear later on. 

• Modify Memory (hence Aranwen not remembering anything that just happened) 
• Charm (stopping Aranwen from attacking) 
• Sending (conduit for Saeriel as Morgana's proof of her wellbeing to Aranwen) 
• Bestow Curse (targeting Aranwen's wisdom with the intent to leave her vulnerable to her spells later) 
• Suggestion (???) 

When they were disrupted, Aranwen and Saeriel latched on to Sending, the spell that was connecting them. The others were abruptly broken, and the residual magic energy went into the 'thread' of Sending, which magnified and altered its effects to what I listed above

Thaaat was a hefty beast--and a risk, putting so many spell eggs into one puppet basket.

If it wasn't interrupted, that would have been very bad... 

...most of us mightn't have made it out had the spell(s) taken completely, I'm guessing.

Thank the gods that Morgana "overreached."

90% of our characters' problems can be solved with a quick stab of a shadow creature.

The other 10% with therapy.

I think that math is right.

...I'd let either or both of you stab me should it save the lot of us in the end.  

I'd let either of you stab Eol too