"but just questioning the need to lower it at all during step 2 of the feather. after all the man"
It's the difference between the poise. As man, swinging backwards and up in something more comparable like a back feather, you can have a slight net forward poise with your partner swinging up into that space. Think if you tip your refrigerator slightly forwards, the back comes up - ditto the man's heel during that rise.
It's different for the lady going backwards though. She is in effect poised forward in the lower body, but shaped backward in the upper spine - the net result being more neutral. In some wyas she's over her partner on the upswing, but in others he's still over her. If you ignore that internal detail and just model her as a refrigerator, to stay neutral overall means you can't tip her forward, so her heel stays down.
"man can take three subsequent backward steps without lowering he heel (eg 3-5 of a natural weave)."
That's a different kind of up. It's not really "swing up" but more "tiptoe across the floor" up - well, actually more "ball of foot across the floor" up. A lady doing the same thing would not put her heels down either.
Ultimately the difference is that in the swing figures, one person is on top of the swing and the other underneath, and the details of how that is done are different depending on which person is on top. In the weaves, nobody is really on top, instead you are both at what I would call "weave height"