Move the Join Party in Progress checkbox back to where it was

[Niwashi]

You also cannot easily see if the "in progress" box is checked

It's shown on the top bar above the main window (next to where the "settings" button now is). Admittedly, though, it's shown by icon so you need to learn what the setting icons are. (This one is a yellow arrow pointing to a blue party flag.)

and additionally it doesn't save the state, as in it unchecks itself after you join a party or something

This part I agree should be fixed.