Yes you need to experiment with combinations. Remove KB/Mouse and just hook a USB pen Drive + SSD and check what happens.
Okay, removed keyboard+mouse, hooked usb pendrive and SSD, it goes to BIOS again.
Is your KB/Mouse hooked to USB3 or USB2 if 3 try with 2.0 ports. Some boards still offer 1 PS2 and 2 USB2 ports.Also if not yet tired, try connecting it to front USB if you have one.
The keyboard+mouse was hooked near the PS/2 port at the top, yes its 1 PS2 and 2 USB2 ports. Tried connecting to the front, and also at the rear at USB3.0 ports. Still no luck.
One more thing, in BIOS experiment with xHCI handoff settings. Also I hope Windows 10 option is disabled in BIOS since you have installed Win7.
Disabled xHCI handoff, Enabled Legacy USB support. Yes, Windows 10 WHQL support is disabled. No, i haven't installed Windows 7 yet. The technician loaded Windows 10 when they assembled the system.
I guess it's time i take it to the technician. I know it will be hard just to guess from the internet without physically looking at the system, so i thank you very much for your patience and time and expertise.