wavy mask or wavy feather mask

The Sine Wave iExpression can create a wave path along any other path. Of course, you could then apply the normal feather to this wave path, too.

I'm not clear what you are asking. You need to show an example. and no, AE doesn't necessarily apply feather uniformely. You can have individual feathering withs across the perimeter of a mask. You may want to read the online help on that. I doubt that that will solve your issue, though, so likely you'll have to employ another technique like e.g. a line with a Ripple Displacemnt effect and a blur converted to a circle using the Polar Coordinates effect.

Mylenium

feather *