The graph shows the values reported by the accelerometer (in units of g) at various IMU chip temperatures (about 10° above ambient) with a linear trend line courtesy of Excel. The x-axis looks great – virtually zero drift against temperature; ironically although the z-axis looks awful, the combination of this, plus the 4th order, 0.1Hz Butterworth gravity filter means that height is measured accurately, and is sustained throughout an (albeit short) flight.
But the y-axis offset does drift against temperature, and the spread of samples is quite, well, spread compared to the x-axis. This may well be the primary cause of the horizontal drift shown in the last couple of videos.
I think the next step is to move to the local park where I can check by sight whether the drift is of fixed velocity (which rules the above out), or fixed acceleration (which backs-up this speculation) without worrying about walls.