Would a Feruchemist actively storing Identity be more susceptible to Forgery? Would more outlandish changes be able to take effect? Thanks for your time, and have a wonderful day.
Yes, if you store Identity, it makes you susceptible to ALL KINDS of things in the Cosmere. Forgery would be on the short list.
Does the difficulty of affecting metals in a body with Allomancy have to do with Identity?
No, more to do with the fact that most people are innately Invested in the Cosmere--and certain planets have extra Investiture. Something Invested is more difficult to transform/move/etc with another form of Investiture.
That is what I had originally thought before you capitalized "ALL KINDS." Is Soulcasting people like Jasnah Kholin did doubly hard since people a have a strong sense of Identity and have innate Investiture?
We're getting a bit far on this course, so it's time to pull out the RAFOs. I don't want to overplay my hand and leave the books without anything to talk about. :)
Does that 'inside a body' thing work on most magics?
For instance, if Han stuck Luke into a Mistborn Tauntaun (a distant and unlucky relative of the mistborn llama), would Luke be protected from both the cold and emotional allomancy?
He'd have to get him inside a living one.
It does work on most magics, though the interactions can be odd unless you know a lot about the workings. Emotional Allomancy, for example, works by lapping against the outsides of someone's cognitive self, influencing you the way music might stir your soul. So being inside a living body wouldn't necessarily stop it--you'd just have more interference. Kind of like how you can still hear music outside if it's loud enough.
Actual mind control in the cosmere requires you to get INSIDE the soul, which you've seen happen frequently enough. There has to be a gap or an opening.
Or, conversely, you just have to be so powerful that you can push through the interference.