I tested Nico's method for you with a Debian bios install, but it might be different for the Manjaro live usb.
To stall grub from booting, you can press the left arrow key repeatedly, assuming that doesn't mess with tianocore loading first. Then you press e to edit the boot parameters. For my debian/grub2 install the kernel line was 14 lines down, so you just press the down 14 times. Then press end to go to the end of the line and add a space and iomem=relaxed and press ctrl+x or F10 to boot.
As for rescue systems with flashrom neither grml (debian based rescue usb) or the old gentoo based systemrescuecd worked for me without adding iomem=relaxed. Most live systems seem to use a udf/iso setup making it hard to just edit the kernel command line directly. The old systemrescuecd actually uses a fat formatted partition and would let you edit it, but it is more complicated to set up then the usual livecds.
You could also probably just chroot into your primary manjaro install from your live usb and then install the efi version of grub2 and use your extra usb as the boot drive, possibly copying over your /boot directory as well.
Of course, just installing a full system to the extra usb with a ufi bootloader like you said you already did should have worked as well. Does tianocore use/respect using boot/bootx64.efi? If so, you could copy grubx64.efi which should be on your efi partition, probably in a directory called manjaro, into a new directory called boot and rename it to bootx64.efi. See https://wiki.gentoo.org/wiki/GRUB2#Alternative:_using_the_default_UEFI_firmw... or https://wiki.archlinux.org/index.php/GRUB#Default/fallback_boot_path for more information on that.
That seems like a lot of rambling. Hopefully some of it helps you though.
Branden