The basic way to ''responsive''
I wonder about the basic operation, the way that the elements: text, image, asset like the Lite Box and especially the spaces separating them. In my opinion, these elements should always have as a vanishing point at the upper left corner. Because, this is how the web browser works, right? So the elements should move and reduce in percentage considering the pixel reduction of the breakpoint.
Let me explain (the number are fictitious):
1920 pixel = 100%
1600 pixels = 80%
So Muse should calculate the ratio. 1 pixel is worth how much percentage if the starting point is 1920 pixel. Being done, the images, the text and all elements also the space between the elements should undergo a 20% reduction on the breakpoint of 1600 pixels. Of course Muse could round up to the nearest whole unit: 4, 8 pixel would be rounded to 5 pixel, the 0.2pixel would be subtracted from the spaces between the elements. That way, if some text would be too small, I would have no problem reworking them manually at the specific conflictual breakpoint. It would be simpler than to have to manually fix all the elements at each breakpoint.
Also, the option to copy the position and dimension to another breakpoint would no longer be relevant. On the other hand, it could be replaced by the option to create character style to a specific breakpoint. Look like: select all the text on the page and save the new style of charactere by giving it a new name, type: H1 formt Tablet. As the find / replace option and save a new style of character.
I don't know if I am understanding all that correctly, but if I may, here's a suggestion on your percentage/pixel base responsive solution:
Their could be an option that could be put somewhere where you could enter the minimum and maximum percentage/pixel and font size so that the percentage/pixel base responsive solution would stop reducing or increasing the content/containers/text/etc when it reached that point.