yes, you are on the right track..actually, if you take your basket out of the "pump", you might be able to actually see the impeller..like a big plastic fan with fins (it would be right in that bigger size part of the pump housing just in front of the motor..that has cooling vents usually that cool the impeller on the shaft..where the impeller attaches to the shaft can wear out. (some use a keyway or groove) so that the impeller can make noise and slide back and forth a little on the shaft)

and you're right..if you have... and close the valves..(mine have red handles by the way..the ball valves as mine are called are pretty common you'll see on many pics) and shut off the multivalve by putting it to close (in effect..having the same "close the valve effect on the other side of the pump) then all that's left for water is what is in the plumbing and the pump that is between the the multivalve and the other valves..so yeah, you could well have a couple gallons of water. if you take off the pump, just carefully take it to wherever you can dump it by keeping it level of course..no point spilling anymore inside your shed or whatever than necessary. it should have a drainage plug or 2 on the pump so when you don't have to try to tip it upside down or anything..just put on concrete, take out the plugs and let it drain.

some people will try to drain it dry by simply running it with the drain plugs open.i cannot advise such a way.