Hi! Thank you for trying Mosaico!
1. Yes this is a known problem and it will be fixed in the next update (after current 188.8.131.52)
2. You can enable manual arrangement on the fly by deselecting its button (the pencil button) and pressing CTRL+ALT while dragging any windows. In the next releases anyways, there will be an option to make it available in this mode by mouse too.