Let us not over complicate this. When a nest is expanding the amount it will expand is determined by the amount of brood there is.
One full frame of new space is needed per hand sized patch of brood seen. A frame that has a hand sized brood patch on both sides equals two frames of new space needed. A frame that has hand sized brood patch on only one side is one new frame of space needed.
When adding a second box of all new foundation or foundationless frames the method to maintain momentum and not set them back is to move the heart of the nest, the centre 4 frames, as a block, up to the centre of the second box. Do not disturb the nest integrity of those frames. Do not separate the frames. Keep them together. Push the other drawn frames in the bottom box together. This re-establishes a useable nest space in the centre of the bottom box, below what has just been moved up into the new box. The queen will finish filling out those 4 frames that were moved up. When out of space to work she will naturally move down into the bottom box. The bees will draw comb and fill the outer frames in both boxes with nectar/honey, starting in the top box which pushes the queen down.
If you have frames of drawn comb in the second box, then it is not necessary nor advised to move any brood. Just have the drawn comb in the centre of the box and put it on as is.
No, do not ever "open" the brood nest frames to put foundation/foundationless frames into it. Draw out new frames on the outer edges, the sides, of the nest in positions 2 and 8 of a 10 frame box.