Aren't most of these 3rd-party Youtube apps vulnerable to policy changes by Google ... like Protube app?
True. They can work now but maybe no longer in a week, month due to changes at Google/YouTube and that third parties need to implement this in their apps (if they even remain in the App Store). Ever since ProTube had to remove their app from the App Store chances are slim to stay alive as a third party app especially in Apples App Store.
But for now an app such as Musi can be a nice alternative with option to remove ads (pay once) and background playback, Slide Over/Split View (however not working great as far as that last point to be honest). However no support for PiP, Chromecast or offline for that you need to have the paid subscription with YouTube Red (if even available in your country).