Try greenland paddling and try stand up paddle boarding and see what it feels like for you.

The beauty of greenland paddling is that it's gentle, and really you do it however it works for you - if something hurts or the motion is bad for you, you just don't do it. And that's pretty much how it's taught - do what works for you. It's a smaller blade, so you just do it at a higher cadence, like small gears on the bicycle.

By arm swinging back, are you saying that your arm shouldn't swing back behind your shoulder or that your arm can't be extended out at all?

If you mean the latter, just ignore what I say next.

Because if it's the arm shouldn't swing back behind the shoulder with the elbow out though, that's not something you should be doing - it's a torso rotation - your arm shouldn't go behind your shoulder. Your torso should physically rotate like you're looking to the left or right when paddling.

A good way to stop rotating with the shoulders is to take a paddle and stick it through the shoulders of your pfd in your back, which will stop you from rotating at the shoulders and force you to rotate using your torso... Another good drill is to take a paddle and paddle with it without bending your elbows at all - the only way you can complete a stroke is if you rotate.

These images might help - they're basically sideways to the boat at the end of the stroke...