How to get content aware fill to only apply within my mask and not all other transparent areas?

I don't believe that Ultra Key comes across from Premiere if you use Dynamic Link to replace a shot with an After Effects Composition so you must have nested (or rendered and replaced) the clip before replacing it using Dynamic Link.

If that is the case, then Here's the workflow:

1. Add a solid below the footage layer in the Main comp Premiere Pro created. That would be your Layer 2 in your video sample. The new solid will fill all transparent areas. 

2. select the pen tool draw any closed path you choose on the background layer and set it to subtract just as you did in the footage layer.

3. You have the Parent Links column visible so all you have to do now is press the m key to reveal both layer's masks and then drag the pickwhip from the bottom layer mask to the footage layer mask. 

I just threw extract on the footage and added a dark red background solid. Then I animated a mask on the fotoage layer, created another mask on the layer below, then tied the mask on the background layer to the mask on the footage layer.

Now you can generate your fill layer. When it is complete all you have to do is turn off the background layer.

That's about all there is to it.

If your CAF does not give good results let me know and I will point you to some tutorials on Mocha AE.

You have two options. Temporarily turn off Ultra key or put a solid below your footage and use the mask to poke a hole in the solid.

I would probably just use mocha to track the front of the can and then corner pin in a little patch. I think it would take about half the time.