The hardware is not designed to be able to do this. The only software access to the signal path is at the output of the preamp, at the unity gain inverter that drives the ADC, so while you can shift the wave up/down, any DC applied to the input beyond one screen’s height will clip at the top just the same, so if you shift the wave down you will just see the clipped signal.
This has already been observed with some devices that clipped at a lower level than the top of the screen, and required the ADC “window” shifted down to hide the clipping.
Likewise, using ChD’s A-B mode with what I presume you mean is a DC signal on the B channel balancing the wave on ChA, if DC on ChA is large enough it will still still clip the waveform at the same level (roughly at the top of the screen), even though it may appear at a lower position. The A-B mode is mixed by the software, after the ADC has processed the signal, so it has no real effect on either ChA or ChB’s DC offsets.
To work properly, DC compensation for this purpose needs to be done at the input, so that the preamp doesn’t “see” it, and there is no provision for this in the device. Otherwise the very small dynamic range of these devices, limited to just about one screen’s height will just not allow it.
This is a compromise that must be made to keep power consumption down when running on batteries, voltage levels must be kept low, limiting dynamic range.