If you disable Legacy USB Support, you will not be able to boot from the USB device. So enable it. There are no two ways about it.
Redo your YUMI from scratch again. Choose to format the pendrive to FAT32 during the process.. This time include only the Win7 install ISO. ( You can add anything else later.)
Make sure that the ISO file name contains no spaces.
Once you create it, use the onetime boot menu to boot from the pendrive.
Are you able to get the YUMI Boot menu and choose Win7 install?
Read through the Known issues YUMI - Multiboot USB Creator (Windows) | USB Pen Drive Linux
and take corrective action as indicated therein in case you encounter problems mentioned therein. 1. You should make the ISO contiguous 2. During the Windows install process, you should plug out the pendrive, before the first Windows start.
For creating only the Windows 7 installation pendrive, I would recommend that you use Universal USB installer Universal USB Installer – Easy as 1 2 3 | USB Pen Drive Linux
which is much simpler and devoid of any issues.
You may experiment with YUMI at your leisure later.