Dear Benoit,
Thank you for your query, patience and perseverance with this, many thanks to Naveen for all his work on it, and sorry that this seems to be such a hard nut to crack. Also, sorry that the documentation is not better.
Do I understand correctly that your technical issues have been resolved and you are just waiting for answers from Autodesk on your remaining questions?
Here are attempts at a couple of partial answers for you:
> Does Autodesk have any idea why this warning shows and cannot be dismissed?
I suspect probably not.
> Could it be a change in Revit 2019 vs Revit 2018?
Yes, of course it could. A reproducible test case would be useful to nail this down.
> Do you have documentations on warnings and errors?
Not that I am aware of. The resources pointed out above seem to hint that the kind of documentation you are searching for is meagre.
> How are the warnings/errors addressed by Revit? What makes a Warning a warning rather than an error? Severity? Why is that that we have warning messages that cannot be ignored?
I can pass this one on to the development team for you and see what they have to say.
> Could you provide a screen snapshot of the warning you mean?
In order for the development team to analyse it in any depth, they will need a reproducible case:
https://thebuildingcoder.typepad.com/blog/about-the-author.html#1b
> Could Autodesk open the Warnings in the API so we can address them?
Yes, of course. That is a question of priorities decided by the product managers and their teams.
In order to bring such a wish into discussion, please submit it in the Revit Idea Station and ensure that it gets many votes.
I hope this helps.
Best regards,
Jeremy