You could find a cheap old copy of CorelDraw Essentials or WordPerfect Family Pack on e-Bay to get a legitimate license to use hundreds of Bitstream fonts including Futura. Otherwise, there are plenty of free look-alikes on Dafont.com etc. For YouTube videos, the highest quality is unlikely to be an issue.
I don’t see any problem with that. Fonts would be pretty useless if you could not sell books made using them.
Just go buy some weights of Futura if you want Futura. Everything else are rip-offs. There is no free version of it.
Twentieth Century is quite similar to Futura, and it’s included among the suggestions here.
Try OpenSans instead. That’s a free and OpenSource font, which means you can tweak it to suit your needs using FontCreator.
So, keep on searching for a free font that you do like.