Hi! first of all let me thank you for the wonderfull job you do helping people with our favorite game! Then the real question: it's my opponent main phase he cast a PW (say Chandra Torch of Defiance) that I let resolve, is there a moment after the resolution of the PW in which I can cast a spell (say Capsize) to bounce her before he has the opportunity to activate her loyalty ability? Or does priority goes back to him after the resolution of Chandra?
When a spell or ability resolves, the active player (the player whose turn it is) immediately receives priority again. That player can then cast spells or activate abilities, including loyalty abilities of planeswalkers they control, assuming its the main phase and the stack is empty. This means that it’s virtually impossible to ever act before a new planeswalker is activated unless its controller purposely chooses to do something else. This is why there is nothing that triggers when a planeswalker enters the battlefield, as it would put those planeswalkers at risk.
The only possible way to get priority before the planeswalker can be activated is to make it so that something triggers when it enters the battlefield. The ways to do this are contrived and involve silly cards like Mycosynth Lattice, so don’t worry about them.