Not sure this is the cord I used https://www.amazon.com/gp/aw/d/B091G9VZ5P?psc=1&ref=ppx_pop_mob_b_asin_title
People usually have multiple PSUs running their system. It is very tough to find good PSUs that have enough PCIe cables
Yes, you need the CPU cable plugged into the motherboard, otherwise your CPU won't have power and it can't boot. That splitter should work, but I would only use it on a 3070 just to be safe since you can underclock it to 115w
Don't go plugging random PSU cables into other PSUs. You have to make sure the pinout is the same, otherwise you could mess up the entire rig
PLEASE DO NOT DO THIS. You're taking a cable that was made for 288w and giving it 4 connections that could draw up to 150w max. This is asking to start a fire.
Honestly your best bet is to add another PSU and connect the 2 PSUs with a cable like this: https://www.amazon.com/dp/B091G9VZ5P/
I would suggest a server PSU with a breakout board as your actual best option, but based on your post here I'm going to assume that your PC building knowledge isn't that strong. Dual PSUs would be the safest thing to do.
Please do not cheap out on your power. Stretching your PSU too thin can result in literally burning down your house. Do not put your house at risk over spending another $100 or so on a decent PSU and chaining them together.
Here is a good Seasonic 750w with four 6+2 PCIe connectors for $125