Swing would go at end of porch parallel to joists. Unless im misunderstanding you.

The only thing that matters is the length of the chain. As long as they connect to the swing at the same level, none of the variations above it matter.

How to Install a Porch Swing | how-tos | DIY (diynetwork.com)

Different lengths of chain to make it work so the swing is level.

If the two lengths of chain are different lengths, there won't be much swinging done.

Not true at all. ^

Try it.

https://youtu.be/Vnf_9lcqnnE here's what will happen.

LOL That is a swing, and he is trying to go high. Not the same issue at all. A porch swing doesn't need to do the same thing. :-) Also, the angle of the swing wouldn't be the same. It's front to back, not side to side.

I tried unsuccessfully to find a video of a porch swing with different length chains/ropes to no avail. Why? Because it won't work. You'd end up twisting back and forth just like the video I posted. It's physics. LOL!

Ain't no swinging there....

https://youtu.be/d0zPHh8QqFo here you go. A guy fixing a swing that was hung with unequal ropes, because it was "like a roller coaster".