Is nrF52840 VDDH pin only used for REG0 LDO/DCDC regulator output to generate VDD power for the GPIO circuitry?
or is VDDH power used to power some other internal circuits in the nrF52840?
The VDDH pins are just an alternate power supply, which can be used to set the nRF52840 in High Voltage mode, which supplies it with 2.5-5.5V. Please see this link for more information about the nRF52840s power supply options.
Can I set the nrF52840 into NORMAL mode by connecting VDDH to VDD to a 1.8V power supply?
From the datasheet page 61 block diagram it seems like VDDH only power the REG0 power stage and in NORMAL mode it should be ok to use 1.8V on the VDDH pin even though specified minum voltage is 2.5V. Which leads to my question, is the VDDH power needed for some other circuitry inside like maybe an RF amplifier stage, etc?
There should not be any problem powering the nRF52840 with 1.8V in NORMAL mode. In HIGH power mode the output from REG0 can be used to supply external circuitry from the VDD pin. The VDDH is not used to power any additional circuitry inside the nRF52840. You can read more on the external circuitry supply in section 18.104.22.168 in the nRF52840 product specification.