So, in my experience of working for a ewaste recycling company:
It sounds like you've already taken significant steps to troubleshoot your HP OMEN, but the issue might still be solvable. Given the symptoms, it could be a hardware issue, potentially caused by:
# ***Over-tightened screws***: If the screws holding the heatsink to the CPU/GPU are too tight, it can cause flexing of the motherboard, which may lead to no display output. Try loosening the screws slightly.
# ***Thermal paste misapplication***: Even though you've cleaned and reapplied the paste, excess thermal paste can potentially cause shorts. It's worth double-checking that no paste has leaked onto any other components.
# ***Battery not removed during the process***: While it's not guaranteed that the motherboard is fried because of this, it could have caused some issues with components like the power delivery circuitry. If you have access to a multimeter, you could try checking for any obvious electrical issues.
# ***Resetting BIOS***: Since there’s no CMOS battery, you might try resetting the BIOS by either shorting the appropriate jumper (check the motherboard manual for specifics) or removing the internal battery and discharging the capacitors again, then reassembling.
# ***Inspecting components for damage***: Look closely at the motherboard and surrounding components for any signs of physical damage or burnt areas.
# ***EC Firmware reset***: On some HP laptops, holding down the power button for more than 60 seconds can trigger an embedded controller (EC) firmware reset. You've already done this, but it may be worth trying again after removing all power sources.
If none of these work and given the absence of integrated graphics on the CPU, it could indeed be a motherboard issue, but checking the above might provide some relief.