Kindly remind me to help you with the remainder of Q2 at a later time or on a later day as I am quite tired now and I might wake up not remembering this question.

For part (ii), I give you an idea first; you need to find the normal to the plane ADC in a similar way. The acute angle between the two planes is essentially the acute angle between their two normals. We proceed to use the definition of the dot product once you have obtained the direction vectors of the two normals.

For part (iii), you must realise that the intersection between any two non-parallel planes will always be a single line. It is, in fact, quite obvious here that AC will be the line of intersection between both planes. Usually, for intersection of planes, the line to find is not so straightforward.