Yes, down definitely shifts in bags. Down leaks out but you would have to have handfuls of it before you would notice.

The first thing you should try doing is see if you can shift the down from your feet back up to the chest. Lay out the bag and see if you can puff it back over.

I have a few concerns with your idea. You are trying to put a baffle inside a baffle? Why not just use the existing baffle? Open it up, add down, and seal it shut again. A messy job to be sure, but it sounds like you are already on-board. The way you are doing it sounds like you will end up with twice as much fabric as you need (heavy) and you will end up with cold spots because it will be difficult to keep these extraneous baffles tight against each other.


It also sound like you are trying to melt shut needle holes in used nylon fabric to get it to be down-tight again. I would be concerned you are damaging the fabric to the point it wall just fall apart in the future.