If this is fake science or a 'conspiracy' then who is co-ordinating just about every university etc on the planet and getting them to agree on the overall trend towards a warmer planet? Who are the conspirators and what is their agenda?
It is not an overt conspiracy, rather it is the expected result of allocating a large amount of research funding for the specific purpose of documenting a problem and finding solutions to said problem before and/or in the absence of any empirical evidence that the alleged problem actually existed. At least in the US, this increase in directed funding started in the mid-1970s, when many other sources of non-military funding of scientific research were drying up after the end of the Apollo program. By the end of the 1980s there was a growing population of researchers whose livelihood depended on continuation or expansion of spending on this problem, so the most probable research topics, as well as results, were those that supported such an outcome. Then the UN gets involved and creates yet another bureaucracy (the IPCC) whose funding and existence depends on finding evidence of a near-term problem and the focus shifts even further from science to "marketing" of climate change hysteria. The overall situation is aided by peer review and editorial policy at nearly all of the relevant journals, which makes it very difficult to publish contrary findings.
How would over 150 countries be persuaded to attend a conference on the subject if the science is weak?
Because the conference had nothing to do with science, the conference had to do with politics and (especially) money. A majority of those 150 countries are poor, and stand to benefit (or, more precisely, their governments and/or government officials stand to benefit) from the handouts expected from the more-wealthy countries under a "climate change" agreement. Many other of the countries are developing or aspiring countries that stand to benefit from less-restrictive rules than the industrialized countries, which will improve the competitiveness of their industries versus those in Europe and North America. Most of the remaining countries are the ones who paid for the problem-focused "research" in the first place (and many politicians in those countries are in positions of power due at least in part from support by "green" voters or NGOs).
How have oil companies been persuaded that climate change is a serious challenge?
Because the oil companies have been politically well-connected throughout their existence, and have realized that they are better off being involved in creating the rules for a petroleum-restricted future than being the victims of such rules.
Think about this, would the U.S. government be taken in by a 'scam' on this scale?
Yes! In fact, the U.S. government has a long history of such behavior. A recent example, although of much smaller scale, was their organizing a multi-nation coalition and starting a war due to the imminent threat of Saddam Hussein's weapons of mass destruction. An example of similar scale was the demobilization to nearly pre-war levels in 1946 under the belief that the elimination of the Axis powers meant the end of major world conflict.