What Certificates should be where? GSLB Trust Certificates vs Device Trusted Certificates

First, iQuery is used between the GTM and devices that it wants to gather resource information for. Second, I believe these certs are where they need to be and you shouldn't have to worry about their respective locations. Finally, your GTM HA configuration is a bit much since GTMs are inherently redundant and adding the additional active/standby configuration is a bit of overkill. I would keep the standby GTMs at each site in a storage cabinet somewhere and if your other GTMs ever fail you can swap them out. The LTMs I would leave as is because that is the ideal HA setup for them. Alternatively, if F5 lets you, you can send the standby GTMs back and get a refund because they really aren't necessary. If they don't give you a refund you can install them at 2 other sites and have 4 GTMs worth of redundancy across 4 locations or as I said earlier put them in storage and have them as backups if your live GTM at that respective location.