For part d we must be fully aware that the first instance of getting towards 5cm above equilibrium will occur before the spring ever hit equilibrium. This means we must take our 0.212043 s that we've been working with and add a period to it.
So part d the answer is 0.889 + 0.212 = 1.1 seconds.
Part f tells me that part e is the two stay together. So I will only put thought into part f.
The reason I did this entire thing as a differential equation was this part, actually. We know the force at any given time will be represented by:
F(t) = ma
F(t) = m(x''(t))
F(t) = -157.5cos(7.07 t)
So if we doubled the amplitude our force function would change to:
F(t) = -315cos(7.07 t)
An object of mass < 4.5 kg would not be able to stay on simply due to the fact the force tops out at 32gs!