Hey, I’m not an A350 pilot but hopefully can shed some light on your questions.
N1% doesn’t solely depend on your load, there are other factors which may come into play to determine your N1% such as airport elevation, runway length, specific procedures at an airport flap settings etc. In the A350 specifically you should aim for approximately 90-98 N1% if taking off from an airport with elevation of 0 feet and no special procedures/conditions at Flaps 2 at 75% load.
Depends on your weight and climb profile but 250/320/0,85 is a common climb profile. This means climb at 250IAS below 10,000 ft MSL and then accelerate to 320IAS above 10,000 ft MSL until reaching FL280 where you would fly at Mach 0.85 until cruise.
Engines are more efficient at higher N1%, so it’s usually 80-90% N1 during cruise.
No, you would descend according to the descend profile. A common descend profile is 0.85/300/250. So Mach 0.85 until FL280 and then 300 knots IAS until 10,000 ft MSL below which you’ll do 250 knots.
Hope this helps, any more questions feel free to ask.