Yes the DECA BB will come with the power supply since there is no receiver to power it when it is connected to the router and coax to the SWM splitter to add internet to the coax, the DECA Home adapter has the receiver to power it to split the internet off to feed the Ethernet port on models below the HR24.
Once you have the DECA BB installed reset the HR24s so they will see the DECA BB and use it, you may also need to rerun network setup.
Right, Whole Home won't work with Ethernet cables connected to the HR24s and yes On Demand will work.