Video A on layer 1, PNG sequence on layer 2 above the outgoing part of layer 1. Add Track Matte to layer 1, select layer 2 as the matte.
Copy layer 2 to layer 4, so that they are at same location in sequence. Add incoming video B to layer 3. Add track matte to layer 3, select layer 4 as the matte and revers the matte.
I’m not sure how your matte works, so you may need to revers the outgoing and not revers the incoming. But that should do what you need.
Good Luck!
TC