Do the instructions give a maximum pressure rating?

The recommendations seem like a complete botch job. Some clearly only apply
to mains pressure systems whilst others only apply to gravity fed ones.

For example, a tank fed system wouldn't need the non-return valves, as the
tank air break is sufficient to meet water regulations for contamination of
the mains supply.

I think the restriction for the cold feed coming from the tank is an attempt
to ensure that your supplies are balanced and they are assuming here that
you have a gravity fed system, despite earlier demanding non-return valves
that a mains system would need.

I have not seen a modern tap that couldn't cope with normal mains pressures.
I'd just connect it up to the mains cold supply, assuming you have either a
pumped or mains pressure hot supply. If you have an unpumped hot supply,
then I would fit a pump. An unpumped gravity shower is like trying to get
wet in the Sahara during a particularly long drought.

Christian.