First: "Upward mobility is one of the most important thing for a government to provide." What do you mean by upward mobility? Why would a government provide it? Why should providing it be reserved to the government?
I don't think providing it should be reserved to the government, just that it is an important government role. Providing means of upward mobility -- education, training, healthcare and support -- is often not well suited for the free market. The free market often caters to those who can pay. People that need the most help attaining upward mobility are often people who cannot pay.
Second: "I think we can all support the goal of a true meritocracy, and that simply requires us to provide lower class citizens with more opportunities to attain upward mobility." First, what is an opportunity to provide upward mobility? Who are the "us" who are required to provide it? How are those people going to be compelled to do so? (Or, who shoots "us" if "we" don't?)
A meritocracy is a theoretical concept where everyone gets exactly what they deserve, based on merit alone. Currently, many do not get what they deserve because they find themselves in situations outside their control that limit their ability to move up the social ladder. Someone could be a genius music composer, but have a costly sickness that has him stuck in a few low paying jobs just to pay the med bills. Someone else might be very intelligent and mathematically inclined, but coming from a poor family does not have the opportunity to go to college. Most people would consider these situations a failure of a true meritocracy. On the other hand, many people have inherited wealth, or just come from wealthy families that can pay for a fancy diploma, and so find themselves in lucrative jobs despite not being all that skilled.
I think that providing upward mobility, and limiting the consolidation of wealth is a useful task for a government. I can see you have some anarchist/libertarian leaning beliefs, and I respect that. But I think that large governments can accomplish things that smaller groups can't. In my perfect world, taxes would be heavy, and they would fund social programs in education, healthcare and other social nets. I know some people find taxes distasteful, but I often find that that is because they believe things already are, more or less, already a meritocracy. I don't subscribe to this point of view. It seems clear to me that a persons social status is mostly inherited. I find that more distasteful than taxes.
So the 'us' is the general public, through taxes. And I believe it would be the government that shoots you if you fail to comply (or more likely imprison you). I will always trust the government long before I trust corporations. But I admit, is isn't a good idea to trust either very far.