As I understand it and I should say that I do not use the Nvidia web drivers and most of my builds have UEFI, you have several issues here :
In order for your unsupported GTX 750ti graphics to work you must use the web drivers.
In order to use the web drivers certain information must be retained in NVRAM.
If your motherboard does not natively support NVRAM then it must be emulated (The problem 6 solution).
Emulated NVRAM is not supported when Clover is installed in legacy mode.
Therefore your first task is to determine if your motherboard natively supports NVRAM.
If it does you can proceed with troubleshooting to find the cause of your problem.
If it does not then your only real recourse would be to change your graphics card for one which has native support.