I’ve found with a portable shower that 6L is plenty enough for one person based on get wet, shower off, soap up and shower on rinse off.
So I would say a) 2.5bar 14L/min is what you need as it’s unlikely you’ll be using the tap and shower at the same time. No reason why you can’t use the 18.9L/min pump but you may find that more water is wasted.
2.5B is 25m of head so that‘s plenty.
Whale state that 16L/min is enough for a shower, they are probably talking boats / motorhomes with much bigger tanks than a T6.
The greater the head (height) between the pump and the outlet the lower the pressure and flow rate.
I use a Whale GP GP1002 pump for my portable shower with a pump rate of 10L/min at zero head, 8.75L/min at 1m head and 5.75L/min at 3m head. I probably use my set up at 1m head and while it is not powerful it is perfectly adequate.
Remember that the longer the wiring between the battery and pump (both ways) the greater the DC voltage drop, reducing pump speed and flow rate.
