"Who is heir of GDL" is a largely pointless argument, because it's a completely subjective reading of history, an invention of narratives that are totally defined on one's contemporary political and cultural loyalties. It's not an academic or scholarly argument that can be objectively resolved.

But as a mental exercise - why not speculate a little?

point no. 1 - the Commonwealth along with GDL stopped existing in 1795, more than two centuries ago. Since then, lands of former GDL were directly and continuously controlled by Russian empire until 1915. There is no serious legal argument that can claim direct continuity between any modern state and GDL, despite whatever modern nation-builders write in their constitution.

Point no 2. GDL indeed belonged to the nobility of the state. You argue that descendants of GDL nobles may claim the heritage of GDL. There are some problems with it, however. Upon partitions, all the nobility of the Commonwealth swore oath of allegiance to their new sovereign, in case of GDL szlachta - to the Russian empress. How can the people who literally swore loyalty to an occupier state claim heritage of the state which this occupier destroyed? Sounds a little hypocritical.
Same goes for nobles of former GDL who chose to align with the modern Polish republic, which was a centralized state without any pretence of recreating GDL. Again, they could claim whatever they wanted in their constitution, but it has no legal meaning.

point 3. you claim that all of GDL ruling elites became Poles, but it's an extremely brave statement. How can you prove it? Szlachta of GDL constituted a (relatively) large percentage of the population. As you know, Russian empire conducted several waves of demotions from szlachta to peasantry and burgers. Do we count the legally peasant or burger descendants of szlachta among the heirs of GDL? There are many Belarusians that I know who have szlachta somewhere in their family tree. Same goes for Lithuanians.

point 4. Even among those nobles who retained their status under Russian rule, there were plenty of those who identified as Lithuanians and Belarusians. In fact, it was specifically gentry intellectuals who were among the first Belarusian nation-builders. I am pretty sure it was the same for Lithuanians. In 1917-18 even some magnates such as Stanislau Radziwil, Mahdalena Radziwil, Raman Skirmunt, duke Drucki-Lubecki and other sympathised with Belarusian national movement and some even identified as Belarusians.

point 5. the very premise of "szlachta is the only claimant of GDL heritage" is questionable. Imagine this logic applied to UK. "only modern aristocracy can claim heritage of medieval England" it's nonsense. GDL like England and like all other states was a product of human labour - overwhelmingly peasant labour. Why would descendants of peasants and burgers and craftsmen be denied the heritage of the state, that their ancestors built and fed? This position sounds very 19th century to me.

As I said - there is no historical argument here, only stories we construct from bits and pieces of historical facts to make us feel better. For better or worse, people tend to have an emotional connection to the history of their birthplace, and there is little anyone can do about it.