Questions about the Belarusian position
I'm not quite sure about Belarusian ethnic lands - could you be more specifi, because we do have opinions that it was Lithuanian ethnic land, and also that it was Polish ethic lands. If the territory is defined by Russian Empire state sponsored linguist Karsky theories, it is no wonder indeed, especially knowing "there are no Lithuanians" stance taken by that state. We could discuss this at someplace else, as this is very complex issue.
Further - as far I do remember, Belarusian Rada of BPR did recognize Lithuania's Republic rights to Vilnius and Gardinas. It was contemporary decision, it might have changed later.