Here are the answers to your questions…

What exactly do you mean by “independent power”? In order to avoid ground loops, you should try to have your Amp/monitors, guitar pedals, and computer all connected to the same power source. ANS. I have tried running my laptop from internal batteries and by USB PSU.

ANS. By ‘Same power source’ do you mean the same 125v mains outlet? If so, that is what I am doing. Clearly the laptop has a USB PSU, The H90 has it’s own PSU and the AMP has an internal PSU. When I tried the splitter I also tried connecting a usb psu to the PSU lead (see below).

How is the H90 being powered? ANS. Via the Eventide switched mode PSU supplied with the pedal.
How is the H90 connected to your amp? ANS. Via a 1/4 mono male to male cable.

Have you tried using the H90 with no other pedals in the signal chain? ANS. Yes, the noise is injected into the signal out path regardless of whether that goes directly to the amp or to the next pedal.

A USB hub vs a direct USB connection shouldn’t make a difference here, unless the hub is advertised as some sort of ground loop isolator. A direct connection is best for troubleshooting the issue further. ANS. OK, I am now using a direct connection USB between the H90 and the laptop. Unplug, audio is silent, plug the USBC in and it’s as noise as hell.

Which isolator did you try? ANS. iFi iDefender Max – eliminates All Distracting hum and Noise in Your Computer Audio System – Enhanced Resolution and Dynamics – Compatible with USB 3.0
What do you mean be a splitter? Please be precise. ANS. MOGOOD 60W USB C Splitter Cable, 2-in-1 USB-C Male to Dual USB-C Female.