UltSans: "To be honest... I think Fandom is still stupid as fuck. Either way, good drawing, but over exaggerated their personalities."
Zarethra: "... I feel offended."
UltSans: "Alright, so... Here are my comments. About their personalities, are terrible, very fandom. About the gifts, kinda meh, so I prefer stay out. But what I want to clarify.. is about..."
All then went blank and UltSans standing while holding a bra with his hands.
UltSans: "Yeah, sure. The main function is to hold the breasts, but seriously, everything has multiple uses depending of the situation and necessity."
UltSans then started wearing it.
UltSans: "Me, personally, I ain't using it, but yet, this is just a demonstration. As you can get extra pockets to save stuff, as pocket knives, cash, or anything small that you could need. Bras can be very handy, but seriously, what a waste of brain cells to watch that shit."
UltSans took a deep breath while shaking his head, and then quit the bra to then burn it and make it gone.
UltSans: "And about the last part?... Well... Nothing new, but kinda crazy that rocket, I would personally love to see that explode. I always enjoyed good fireworks and explosions."
Explanation
He holds and shows the bra, because is to show it directly that is holding it and showing it.
Explains the main function to clarify what everyone supposedly knows a bra does, in case anyone doesn't actually understood properly the use of a bra. Something to hold the breasts, not be used to assist any exhibition or superiority, but as a tool to assist the women.
Lists the alternative uses, to show is way smarter than most in the fandom, the characters, and how his vision is of the objects and items. Seeing every thing and how he analyzes them to see the value and possible uses. So is to claim is way smarter and more refined than anyone in the fandom and the characters, more intelligent and analytical, careless to use a bra when is a man and not a woman and still using it for a demonstration, and how practical things can be if used properly with a clear objective of use. In the comic, Chara doesn't have any chest size, so he uses the bra to laugh in those claims in a calm indirect way, and showing his superiority and also to teach.
He then gets rid of it, because doesn't want to use a bra. Is a man, doesn't have any actual use for one, and doesn't want one, so got rid of it.
About your point 7, is correct. For him, that was pointless, boring, and completely useless. Something more of torture than for enjoyment.
He burnt it because used magic, being calm when was irritated, a way of showing it. And then "gone" because is his personal void, and in there he made out of thin air the bra, and can also make it disappear as made it appear.
Zarethra feels offended because is Chara after all, and yet, finds offensive how over reacted Chara in the comic is for the chocolates.