1. I think the reason they will give is that an infected person has contacted them to say they were within 2 metres of you for 15 minutes. You won't necessarily know who that is because the contract trace system is supposed to guarantee anonymity.
2. I could be wrong but my understanding is that it is only the initial person with symptoms who will get a test. The people who are contacted, who may be asymptomatic are not guaranteed to get one.
3. If you haven't been given a test and don't have any symptoms but are nonetheless being asked to self-isolate, what will you do?