Google says, that all I need to do is simply go to Arthorn Windsong and she'll replace the lost whistle, however when I talk to her, there is no such option.
So, that is that. Without the whistle:
can't pick up next quest,
can't abandon and retake it, see I don't even have it,
can't get Anzu's doppelganger as my riding *****, which was the point all the time.
I may well be missing something, however my firm believe is that I pictured you guys the whole painting.
Any feedback will be appreciated, well if it's useful.
Happy fixin'.
Edit: Bug report created as Islingr suggested. Thanks for the idea.
Edit 2: Bug report confirmed, however there are some misunderstanding in current interpretation:
This is the entire problem - fix this, and all will be well. Use Create Replacement Whistle as dialog option.
2.This is incorrect -
Arthorn's Sparrowhawk once summoned with whistle and turned quest in, does not provide follow up quest to complete chain.
Arthorn's Sparrowhawk does provide follow up quest, I was reading the quest down, going to press accept, but the quest giver disappeared before I do that. When you summon him with the whistle to turn quest in, he takes 2 quest items - the one you grind, and the whistle itself. Then you accept next quest, which provides the whistle anew.
I'm stuck between this two moments, this is why Arthorn Windsong should have gossip/dialog option to replace lost whistle -> you summon Arthorn's Sparrowhawk -> take next quest to continue the chain.
Hope this clears things up.
1. is there a bugtracker entry about this yet?
2.a if previous is 'yes' then upvote/comment and wait
2.b is previous is 'no' then create with relevant information and wait