As I mentioned before, to pin point exactly where the holes are is to wait for heavy, really heavy rain and you can see areas that are flooded and you have to work under the rain just make sure there is no thunderstorm as it is dangerous. Fill up these low areas with the sands from one hole to another.

Simply scattered the sands wont work as you already have grass and the sand will not shift and move by gravity due to the grass leave and roots. If you think what your neighbor said can work you can try it out by simple do it on one small area to confirm my doubts. At the end of the day area you added sand will be higher, the areas you spray with water will not move unless you water shoot them and it will be empty by the shooting water pressure. The only way to shift and move added sands wtih grown grass is to flood it and disturb the sands with hands a small areas by areas.

Try tying up the garbage bags so that the garbage men will not be able to see what is inside. Usually(or in normal practice), they only collect food stuff rubbish, not construction materials like sand, earth, broken tiles etc. To transport these unsuitable material away, usually small lorries are hired to take these away and dump that at some other dumping yard. I'm not too sure about grass though, but usually they should pass, and the rubbish collected should be able to dispose them for you. Perhaps you got one nasty garbage man there.