Two points:
Not all Russian missiles would have to be launched from the east, Russian planes and naval assets launch most of them (we are told) and there is Russian air space to the north over Kaliningrad oblast as well as naval assets that could be in the Baltic even to the north of Poland although I don’t know whether Russia would risk having missiles overflying Poland. Also Russian planes are probably free to operate over Belarus.
Second point, after a missile interception, fragments or partially disabled missiles could in theory fly towards the ground at any angle, the impact point was barely inside Poland’s territory along an uneven border, so that theory is not outlandish.
Anything is possible however. I do not strongly back either side in this and would like to see a negotiated settlement at the earliest opportunity.
An armistice would stop the killing.
And no party would have to give up any territorial claim or right.