Well, Moscow was always a thread to Lithuania
Not at all, untill XV century Moscow was struggling to break the Mongol (then Tatar) dominance which it did.
Moscow was a natural enemy of Lithuania
And Poland, after conquering Lithuania Moscow would inevitably invade Poland and Poland without the wealth of the eastern grain fields and populations would have fallen.
Maybe they were not imminent threat but they were threat nevertheless.
We need to look at Polands situation, in 1410 Poland a relatively unknown and small kingdom utterly massacred the most powerful military in Europe, suddenly Bohemia, Holy Roman Empire, Hungary and Ruthenian states all noticed Poland.
For Poland to move into Germany was too dangerous, despite its considerable military power Poland had only a third of the HRE population, Moscow on the other hand was an emerging power as well but far less wealthy and weaker militarily so Poland chose to expand eastwards, a wise decision that gave Poland 200+ years of utter regional dominance that no one including the Holy Roman Empire could challenge.
Poland did fall but that was due to a series of events, otherwise even in the late XVII century Poland was a power that equaled unified Russia in military prowess.
Yes they did, but you have to remember that at the time we would have carried some clout in the marriage, and would have been their equal at the very least. On the negative side we would probably have become entangled in the internecine conflict between the German sates, and the political chaos that was the Holy Roman Empire. However with a union of Poland, Austria, Hungary, and Bohemia-guess who would hold the greatest sway?
That was completely unneccesary, yes Poland should have entered the HRE around 1520-50 and would totally dominate it but marrying into it was unnecesary.
Between 1450 and 1600 Poland has the kind of military, political and financial leverage that it could enter the HRE or simply support an elector and have its own way, that Poland did not dominate Germany at this time is a grave error that cost us our prosperity for the next 300 years.