Do seams allow the majority of water ingress?

Some of the water ingress is due to the change in the surface tension when it comes into contact with a hard surface.

That is the reason why a tent fly can be considered waterproof at 1200/1500mm however a rain jacket needs an HH rating several times higher than that.

Tents that have the poles pushing against the fabric can suffer from that .

Apart from that , the stitching holes are typically a bit larger than the thread in them so yes that is where you can expect water to come in , that and the zip.

Yes ,seams would be the weak point but unless you have a tent failure, a poorly set up tarp, or you cowboy on a night that the rain snuck in, it shouldn’t ever be an issue so technically if the material was the same a non stitched synthetic might have a slight advantage but in all cases it’s user error not the bag

Thanks. I’ve had a wet sleeping bag pretty frequently. There’s been user error, tent shortcomings,  moving in my sleep, and just using a small single wall tent during heavy prolonged storms.

I’m curious about synthetic jackets as the popular models around here are split between continuous insulation and quilted.

I do have a quilted Apex mummy bag and the seams bother me a little.