As such, I assume the RIAA will try to argue that it doesn't fall into the same bucket that youtube-dl does, since the method of accessing the site is different (scraping the site, vs a sanctioned API)
Licence to Other Users
You also grant each other user of the Service a worldwide, non-exclusive, royalty-free licence to access your Content through the Service, and to use that Content (including to reproduce, distribute, modify, display, and perform it) only as enabled by a feature of the Service.
I am a user of the Youtube service. I read and agreed with the terms, I have watched videos. Thu I am allowed to reproduce its content. The RIAA explicitly agreed I can distribute it.
2) the Service makes downloads available on any browser, it is up to the browser to show it.
3) "If it turns out that a particular term of this Agreement is not enforceable for any reason, this will not affect any other terms."
Physics clearly make this unenforcable.
"Unenforceable" doesn't mean practically unenforceable, it means legally unenforceable. The laws of physics permit most cars to exceed most speed limits. Those laws are still enforceable.
Unenforceable clauses in contracts are things that are either contrary to law or otherwise don't fall under contract law. So a clause that says "if you sign this you must give up your firstborn son" is unenforceable. That sentence just says that "if we screwed up drafting this contract, just because you were able to challenge one clause due to being unenforceable doesn't mean the whole contract is void." It's a severability clause.
However, Safari (not Firefox) is not affiliated with Google. Google has no say on how the content provided by the Service is presented. The Service provides a HTTPS connection. Its influences ends there.
No. It's available for free users as well AFAIK.
Wait, seriously? I had no idea. I would think the presence of such a feature would severely undermine the RIAA's case.