Acer E-15: Cannot boot from USB (UEFI)

Hello, I'm having getting my USB stick recognized as a bootable device (installing windows 7 onto a system that came with windows 8.1 pre-installed, no optical drive)

System: Acer E 15 (E5-571), BIOS V1.15 (UEFI)

What I've tried:
-Disabled Secure Boot

-Created bootable USB stick with Windows 7 iso, via 'Microsoft USB/DVD Download Tool' (on alternate Windows 7 system)

-Created bootable USB stick with Windows 7 iso, via 'Rufus' (on alternate Windows 7 system)

-Windows 8.1 Advanced Restart -> Boot from UEFI USB Device (No UEFI device recognized)

What worked:
-Enabling 'Legacy Boot Mode': USB stick was recognized upon boot and loaded Windows Installation. HOWEVER, Windows 7 won't instal to a GPT formatted drive under Legacy Boot Mode. I've read booting in UEFI mode is the only way to get Windows 7 to install onto GPT.

What I don't want to do:
-Format the drive, change drive mode to MBR to instal Windows 7. Why? Because then I'd loose the factory installation of Windows 8.1