A newly hired tester joins a cross-functional Agile team. During the first sprint, the following situations occur: (i) The tester finds a critical defect but the developer dismisses it as "working as designed" (ii) The product owner changes a requirement mid-sprint (iii) The tester needs to quickly learn a new API testing tool (iv) The team disagrees on the priority of fixing a performance issue Which essential tester skills are MOST needed to handle situations (i) and (iv) effectively?