Can the high current motor change the ground potential level in the H-Bridge?
The answer to your first question is yes. Now regarding the two ICs:

First, with the IR21844 you can set the dead time. In the simpler IR2184 it's preset.

Second, the 21844 has separate ground pins for the driving logic (VSS) and MOSFET grounds (COM), which is probably what you refer to by 21844  being "decoupled". It is certainly advantageous for your high-current application, as this IR FAQ explains:


  with the potential of generating higher -Ldi/dt voltage spikes on COM, later ICs separated Vss and COM to increase the noise immunity between logic and power grounds, so that voltage spikes on COM did not cause the logic ground to dip below zero volts and generate false input signals. In these cases separate Vss and COM pins are provided.


On the downside, the 21844 needs more external parts and probably costs more itself too (bigger package).