Thanks a lot for the detailed response and links to relevant resources. I agree it is complicated topic and I certainly did not want to discus the applicability of specific treaty articles in a volunteer forum. A specialist concluded that the treaty article is applicable and that there is an exception to the saving clause that allows to claim it (for a limited time) after becoming a "resident for tax purposes". However that specialist cannot help with the tax return.
My (naive) view was that I would "only" need to know how to report this income. There is quite a bit of (non-professional) advice floating around that suggests to declare the total income in line 1 of form 1040 and declare a negative amount in line 8 and note the tax treaty and specific article next to it. I guess I need to consult a specialist on the tax return as well.
btw. Thanks for pointing me to section 6114, in my (non-professional) understanding it waives the reporting requirement (i.e., form 8833) for my specific case.
