When to scale down?

How to deal with exercises which you cannot do “fully as prescribed”. If coach tells me to do Alternating one arm hangs and I cannot do all reps as prescribed should I still try my best or scale down? for example, coach prescribes 2x8 hangs in a set. If on first set I can do 4 hangs with one hand and 3 with the other, while on the last set I can barely do 1 each. Should I do it like this, or should I scale down to easier exercise? If I should scale, when should I do that? after the first set? finish all sets but next time this exercise is given scale it down before even attempting?

Other related question - what is considered a break in coach feedback ? If I need to do 10 pullups - I do 5 in a go, stop to brace myself, do another 3, stop to brace, do last 2 without jumping down. Is it considered breaks? Would it be considered breaks if I jump down and back up?
Or if I do 10 burpees slowly (not as fast as possible) is it considered a break ?