RAND published a study about the defence of the Baltic NATO members ...
David A. Shlapak and Michael W. Johnson, RAND Corporation
... in which they essentially repeated what I thought and wrote about the issue for years.
2015-10 The EU should do something about Baltic security
2015-10 The Baltic Sea and Baltic Defence
2015-08 Strategic QRF
2015-04 Oh, really?
2013-05 Bündnisverteidigung / collective defence
2010-05 Defence policy thoughts for (very) small powers
2010-03 The first (and only?) phase that counts
Their conclusion is slightly different, owing to their different position.
I'm an European, but they're Americans consulting (for pay) an American military bureaucracy that benefits from increased size, budget et cetera. Their conclusion is thus that a few additional U.S.Army brigades deployed in Eastern Europe are a fine answer, while my conclusion is that turning German army brigades into quick reaction forces that are REALLY quick would be the ideal solution. It seems we agree that the Polish army cannot provide the solution because it would in a Baltic defence scenario be largely busy protecting Warsaw against the East (Belarus) and North (Russian brigade(s) in Kaliningrad Oblast).
The obvious answers are thus in my opinion that we need to improve choice of roads in the narrow corridor between Warsaw and Vilnius, make German army brigades REAL fast-moving on road (even if this means the tank battalion follows a day or two later) and at least Lithuania should improve its ground forces (especially artillery, anti-armour and battlefield air defences).
The known poor readiness and low ambition about the rapidity of even a designated quick reaction battalion shows that the German army isn't even coming close to excelling at collective defence - possibly a consequence of two decades emphasis on nonsensical great power games deployments. RAND can be excused for looking at pre-positioned U.S.Army brigades based on this background, of course.