The relay can be wired down to chassis earth, as that really doesn't need any amount of current, but yes totally singular line for earth direct from the battery to the pump, if you want to tap off this for the relay earth that is fine, but not needed as sure to be nearby earth point.
I put the relay behind the n/s/r seat upright, as in the metal work behind the seat at top, underside of parcel shelf there is already a 8mm hole there, ideal to secure a relay holder, then when at shows etc, just pop out relay for a bit of extra security.
tabetha