You're essentially correct. In our PicoScopes, the Analogue-to-Digital Converter (ADC) performs the conversion single ended to give ADC values from zero up to the maximum value for whatever resolution is being used. PicoScope 6 then effectively applies a DC offset to place 0V at a half-way value, and from that point on provides a double ended (split, positive and negative going) display. This is fundamental to the PicoScope 6 Scope view, so any further adjustments, such as software DC offsets, are made afterwards and won't preserve the resolution. To preserve any resolution an additional DC offset must be applied in hardware (before the ADC), which is only available in a few of our fuller featured hardware PicoScopes (unfortunately this doesn't include the entry level PicoScope 2205A).
Just to clarify, what I believe you're asking is "would it be possible to incorporate the user requested DC offset into the automatic DC offset that splits the display (giving a display that isn't split in the middle), so that any user requested DC offset (up to half of the voltage range) won't reduce the resolution", and "would it be possible to have, say 20, discrete values (grid lines) that the 0V line can be dragged and dropped to, in order to perform a number of quick adjustments". So, I will submit a software enhancement request, for this, and let you know the outcome. Thanks for the suggestion.