Yes, can leave it empty. If you want to have fun, can put granular carbon cartridge there, it will not reduce the flow, but will help your main filter to deal with chlorine. And VOCs

Surprised flow rate is much less with  stage 4.... Is it brand new?

As for dispenser, problem is most people want cold/hot dispenser. Usually dispensers are easily infested by bacteria and require a lot of maintenance. I found very nice NSF cert dispensers, but they are... 3000 USD and more, must be made of gold. Also found in 300USD range, but I find that even that is too much. Must be around 500-600RM...

Hello. No, Fresca has a sediment filter as stage 1, so it is like an integrated pre-filter. Stage 1 (as well as stage 2) is washable cartridge, 5 micron sediment, so there is no need for a separate pre filter.