PC power supply for 12v 24A consumer, question on 12v yellow wires
You have a 350W PC power supply. That 350W is split across multiple 12V, 5V, 3v3 and all the other votlage rails you have on an ATX power supply. If you read the power supply's technical documentation, it will probably tell you in there what the current limit for each rail is.

So when you say you have 29.2A at 12V available, you're greatly over estimating the power.

The reason to join all the 12V rails together is a best practice idea. If there are multiple 12V paths in the PSU (which is probable) then you are keeping the load even between them all.

On top of that, you're reducing the voltage drop caused by the current through the wires. Often people talk about the current limit of a cable as if they expect the cable to melt. That is very unlikely to happen. Often the cable length will mean you'll have a 1 or 2 ohm resistance, at 1A, that means you're dropping 1 or 2V along the cable, so you're looking at more than 10% voltage drop due to cable resistance. Having two cables in parallel, you're reducing the resistance (and so volt drop) to half the value.

TLDR:
- You have a lot less than 350W to play with
- Best practice means joining the cables to spread the load
- More cables mean less voltage drop