I think you have two options at this point to get back. First, change back to UEFI from Legacy. You cannot change midpoint on an install. Tap F11 at boot. This should start the Recovery Manager for a factory reset back to 8.1. Downgrading to 7 can be problematic as some drivers may not be available. I think your best option would be to upgrade to 10 if the downgrade fails again.
If the factory recovery partition was overwritten by your Windows 7 install go here and download the iso for 8.1. Pick your version, Home or Pro. Use Rufus to create a bootable USB. Select GPT for the partition scheme in the dropdown box. Where is shows Freedos in the dropdown box use iso image. Click the folder icon and browse to your iso file. Press start. Windows 8.1 may ask for a key when installing. All OEM 8.1 keys are embedded in firmware. You can retrieve the key using a live linux disk as shown in the below links. If it does not ask for a key, then after install will be automatically actlivated. You can create a live linux boot disk using an iso and Rufus. I believe you can use the latest Fatdog64. It is a smaller download and should be able to do the same commands. You only need to do this if you do not need to provide a key during the 8.1 install. Fatdog64-701.iso
If you want to do a clean install of WIndows 7 and it is a full retail disk with key, first disable secure boot. Boot the Windows 7 USB flash drive and at the window that shows what language to select press Shift + F10. This will bring up a command prompt. At the prompt type. Note: You can use the same Shift + F10 and the below commands using the Windows 8.1 iso
select disk X where X is the disk number of the Windows OS. If you have one HDD then this is disk 0
clean this will wipe the partition table. WARNING. Once you do this, you will lose your F11 recovery partition
convert GPT this will set the disk to GPT to allow you to install Windows 7 on a GPT/UEFI disk
Boot your WIndows 7 USB flash drive. If it does not boot and you did not create it with Rufus then use Rufus because it will boot UEFI.
I do recommend you go back to 8.1 first. Then create a disk image with software like Macrium Free to an external drive. In Macrium Create a WinPE bootable USB so you can recover the disk image easily if there is a problem. This is if the F11 key fails because the lenovo recovery partition was lost.