These are very good questions. I don't think I can do justice to the answers but I'll try.
1. What is the correct average global temperature? Or. What year/time period average should we use to determine the correct temperature?
Scientists have figured out pretty reliable ways to measure temperature/climate changes over both long and short periods of times. For examples, by look at O18 isotope content in various fossiles. To explain any of these methods it will make this post too long. The key is, whether measurements from different methods and from different regions agree with each other. For the most part, they do. When they found disagreement, it's often factors that were not considered at the time (for example, the ozone hole which leads to regional cooling).
2. Has the Earth been warming? How much? For how long? What happened to the global climate before this most recent trend?
The Earth has been warming in the past 200 years, and the trend is accelerating. The global climate was relatively accomadative to humans in the past 10,000 years. This is the direct cause for the rise of agriculture. Recent human activities are about to break this relatively calm period. Before that, there were violently warm and cold periods (ice ages, for example).
3. Is global warming bad or good?
It's not the warming per se. It's the violence in the climate pattern (more hurricanes, more droughts and floods, etc). Bad or good is subjective. Any significant changes in the climate will lead to significant reduction in agricultural output. Is that a good or a bad thing? That depends on who you are.
4. What has caused the most recent temperature change? Man? And what caused the previous temperature change? And the one before that? And what about global warming on Mars?
Although no science is 100% certain, there is a very high probability that the recent climate change is cause by human activities. There are many scientific studies on this. They more or less reached the same conclusion.
5. If there is global warming and it is bad, how much would it cost man to stop/reverse this trend? Or will the trend reverse on its own? And would the money be better spent on other things?
That's the question for politicians/voters to answer. I don't know whether the trend will or can be reversed, or whether humans need to actively try to reverse the trend. However, since the recent climate changes are likely due to human activities, it would certainly be a good idea to try to reduce human influence on the climate as much as possible, before we understand more about it. What is the cost/benefit ratio and where do we draw the line? I don't know.
Accept the scientific fact of global warming, then we can debate whether we want to do anything about it. There is no reason to mislead the public by either exagerating or denying it.
