Part 1
OK, I'll have ago at this, but bear in mind it is over 20 years since I did A level Physics.
Light exhibits properties of both waves and particles, which is one of the major stumbling blocks in tying down how it 'performs' in certain situations. You can reasonably call them 'lightwaves' as this is how their behaviour is usually explained. So for Q1, you can assume they are of the form of a sinusodal wave (a constant up and down wave like you see on osilloscope in B sci fi movies!). This means the 'peak' is at the top of the screen and the 'trough' at the bottom. This leads to Q5. If we call the peak 1 and the trough -1, you can see the inter action between them. If 2 waves are 'in phase' , their peaks and troughs will add together to get a brighter 'beam'. However, if they are out of phase, the peak(1) and the trough (-1) meet, add together and make 0, or no light. The energy doesn't 'go' anywhere, they just cancel out. Not sure what you are getting at in the start of Q2. Gravity is totally dependant on the mass of an object. However, as the electrons and protons (I think its both) have mass, although very small, they can be affected by a larger mass. During a Total Eclipse of the sun, it is possible to see a star behind the sun appear either side of it as the light is bent round the sun by the sun's gravity, making it appear as though the star is in two different places.Gravity has nothing to do with the slits, this is the slit creating an 'interference' pattern (see your text books for explanations).