1 - Undertale uses the 8BitOperator font, which is open source.
2 - Yeah, it looks similar, but it uses its own panels, dialogue, and even the font for the logo. Undertale's menu is just a black screen with text, which is not unique at all to it.
3 - It's changed, and it's not Frisk.
4 - Original songs, just using the same soundfonts.
5 - There is no "Undertale soundfont". All soundfonts used in Toby's music are from other people, or even other games (Like the THInst soundfont, from Touhou).
6 - Ok.
Some stuff might look the same but it's its own game. There is no assets from Undertale (they have stated that themselves). If you'd like, you're more than welcome to support the game by buying it, but it's never going to be free.