1. The accumulation of GHG in atmosphere due to anthropomorphic activities. (Regarding this issue, it seems almost everybody agrees that GHG is increasing in atmosphere due to our action, 

-> I'm glad that you agree that anthropogenic activities are the reasons for the increased CO2 levels. 

but I think people think consumption of polluting GHGs will go down with time as people's purchasing power goes up and they adopt cleaner technologies.)

-> IPCC publishes the future predictions under different scenarios regularly. So even with the best scenario (

people's purchasing power goes up and they adopt cleaner technologies.) that you mentioned we will have to witness the changes due to the doubling of CO2 in last century. It is highly unlikely that we'll be able to meet this scenario with the current development and investments. Future cleaner technologies are imminent but what  has been emitted, have already been emitted but will have consequences.

2. The impact of GHG on temperature. I don't think this has been settled. Some European studies think that global mean temperature will increase by more than 10 degrees if GHG stock increases two fold, which is just ridiculous.

-> If you take any course related to this, you'll need to do the math on how much temperature will rise with the particular increase in CO2 or other green house gases. It's called global warming potential and it is the established knowledge; again if you want the formula to calculate it, I can google it and paste it here. 10 degrees increase is possible if all GHG stock increase. GHG is not only CO2; there are other gases such as N2O and CH4 which has much more global warming potential than CO2; so if you double all those, it might be possible; I've not done the math yet; if we really want to go into this debate; I can again google it and we can play with the math here. How likely it is going to happen? To answer that, we've to look at how much increase it is happening in the world currently.

3. True impact on people of temperature fluctuations. Nobody really knows about it. After all the world has seen 17 ice-ages before. Temperature may increase or decrease even when people have nothing to do with it.

-> There have been many ice-ages, and there might be another one to come and the current impacts of all GHGs might be neglected. But do you think we should wait this to happen? Currently, we don't know if or when ice-age will happen; what we know is GHGs are increasing and it is a fact that it will increase the temperature and we are seeing the increase in temperature.