im very new to pc building so i do not know. can you power your motherboard with a pci-e cable?
In theory? Yes. In reality? No. There are some boards that have a PCI-E plug (or molex) but those are for the extra PCI-E slots on the board like you would find on boards made for mining where you would have like 10 GPU's hooked up.
In principle, yes, but not without heavy modifications to get all the voltages you need. May as well just use a legit 20/24 pin connector on an actual PSU, since you'd just be going back there anyways.
(There are some exceptions, including certain SFF systems using mini-STX motherboards.)
thanks for the info.