The third D completes the second octave. Which I’d regard as the top of the ‘normal’ range. But, yes, you can get higher.
Low D, high D and top D make sense to me on whistle, but it’s generally clear from context which people are referring to and I knew which you meant.
In all fairness, Youtube is a social media gravitated towards not only showing off your skills, but just genrally sharing what you know and have. There’s absolutely no requirement to be proficient in what you intend to upload. That’s just the convention of the platform, and in my opinion does no harm for anyone to upload newbie-videos over there, and it’s also a great channel for receiving feedback and improving your playing. You must have some sort of media literacy, just like all around the web, to realise what material to take as something useful and what to enjoy otherwise. If everyone followed that mentality of not sharing anything until you have some worthwhile to share, no-one would ever dare to upload anything, being scared of judgement from the ‘real pros’.
A different thing would be, though, to post some newbie-material and claim to be pro and mislead other beginners on purpose. An unfortunate element in Youtube, too, but inevitable. Once more a situation of personal media literacy for the consumer.
Summa summarum: post all the videos you like, everyone is responsible for their personal way of interpreting material from the web! 
I really shouldn’t post before having my morning coffee and go off on one (see above). Youtube is probably a whole different can of worms. But enough from me, I’ll leave it at that.
If uploading to YouTube meets your woodshed analogy, constantly drawing attention to it here is the equivalent of constantly coming back into the cottage and saying ‘please come out to the woodshed to see/hear what I’ve done.’ It’s not necessary, and it is imposing yourself on the forum whether or not people actually choose to follow the links.
A Youtube video you can click on, or not, is no kind of imposition on anyone, and it’s stupid to pretend otherwise. Leave the man alone. He’s doing no one any harm.
See above. If someone wants to fill their YouTube channel with the musical equivalent of T-h-e-c-a-t-s-a-t-o-n-t-h-e-m-a-t, no-one’s stopping them, but we don’t need constant reminders. Some occasional links to new videos with questions like ‘how do I do this?’, ‘am I getting this technique right?’ or even ‘look, I’ve taken this on board and am improving’ might be different, but that’s not what we’re getting here.
In the past newcomers here have put their experiences as a learning whistler in a blog and posted a link in their signature line.
I believe that’s a decent compromise that leaves everybody free to visit or ignore as they see fit.
Locked.