You can't take the rental car one-way because cars we drive on the correct side of the road. A UK (right hand drive) is no use to the rental company in mainland europe (left hand drive). This is why cross channel one way rental fees are frightenting - you are effectively paying a fee to have the car trailered back to the UK.
You can reduce the hassle in Dover very slightly. There is a short stay car park at the check in area in Dover port (eastern docks). So you can drop the passengers and luggage at the terminal then drop the rental car - then only one person needs to deal with getting rid of the rental car away from the port. The address you quoted is about a mile from the eastern docks.
In Calais, the car rental desks are in the terminal building - where the port shuttle bus will drop you.
As others have mentioned, these days the cross channel ferries are designed with car/lorry traffic as a priority - and foot passengers are a small minority of users.
It is a bit of a faff but not too difficult - so long as you're not asking the 86year old to drag heavy luggage.