Hi Everyone. I received my MacBook Pro earlier this week and ran into the same issues, I spent 4-5 hours with Apple Support:
1- Current version of Mojave will not work on the MacBook Pro 18. You need to wait for developer Beta 4, I believe coming up next week
2- A bootable USB will not work and will not install Mojave either
3- If you delete the High Sierra volume via command-r/disk utility, you are toast for now, You will not be able to reinstall High Sierra. Why? Because the driver necessary for the MBP 2018 are not available yet via internet recovery. That should be available soon
4- There is a way to properly recover High Sierra if you deleted the volume(T2 chip uses same process as iMac pro):
1- Current version of Mojave will not work on the MacBook Pro 18. You need to wait for developer Beta 4, I believe coming up next week
2- A bootable USB will not work and will not install Mojave either
3- If you delete the High Sierra volume via command-r/disk utility, you are toast for now, You will not be able to reinstall High Sierra. Why? Because the driver necessary for the MBP 2018 are not available yet via internet recovery. That should be available soon
4- There is a way to properly recover High Sierra if you deleted the volume(T2 chip uses same process as iMac pro):
1-If you have another Mac device like an iMac or MacBook, use a USB-C cable (by apple that can carry power as well). and connect it to your MacBook Pro 18 and other MacBook device. On the other Mac device, launch apple configuration. Turn on your MacBook Pro 18 and enter DFU mode. This will create an ibridge connection allowing you to push MacOS high Sierra to MBP18. The challenge with this is that only a couple people at Apple seem to know the exact key combination on the MBP18 required to initiate DFU. (DFU mode same as iPhone)
I am assuming that something within the next 1-2 weeks, internet recovery will be updated with the drivers and Mojave will be updated to support MBP18. If you can't wait, swap your laptop. After 5 hours with apple support, that's what I ended up doing. Hope this helps.