You should never expect to have rights to anyone's server API endpoints, esp outside of their TOS, which is basically what trying to build your own front-end independent of youtube.com is. However, with most browsers, you have all the rights to build extensions that hide divs and change styling and add new elements with data that's already loaded (as long as you're not calling API endpoints that aren't called by the site's source though, you run into the same problems again) which would accomplish everything you were trying to do.
Of course, now there are pages that detect if their elements or code are being manipulated by the browser (i.e.: Ad blocker detectors).
Thankfully, it's rare because people using ad blockers is apparently rare on the whole.
> Thankfully, it's rare because people using ad blockers is apparently rare on the whole.
I wish this were the case. There's quite a number of websites that use Admiral's services to detect adblockers. Admiral got 19m dollars in funding last year, so I imagine the adblock threat is meaty enough.