Answer 1

put the related components as a group in JPanel, then arrange those panels in a single window.
if this got way too much which is bad for ux, use JDesktopPane but if using "virtual" desktop isnt suit you, last option i could think is to use JTabbedPane
i always use grouplayout i know its not the most readable one, but it just stick to me.

Answer 2

they're equally interesting tbh. gridbag has a big plus in readability and much more easier to comprehend.
group layout in the other hand really easy to get messy and a bit harder to understand, which is normal since it is made to be automaticly generated not hand written. but has much more control over smaller things from my perspective.

Answer 3

oooh so you sepearte each panel as its own class ?
i think simple setter and getter should be good for comunicating between class

Answer 4

i rarely subclassing any swing container, simply because there are no behavior change or addition that i need.

i think setter getter is the best way

other possible way, by using abstract function to pass other panel:
https://code.sololearn.com/cILU0qF33U6m/?ref=app
it may not the best solution though