These are two of the things I want to write my MA thesis about.
I just watched a very interesting lecture by Joseph Nye at Chatham House in London talking about those two things.
In short his answers were no & no. The reasons why were interesting.
Firstly our conception of power is too narrow, we should take into account military, economic and transnational power. Militarily the US will remain the global superpower for some time. Economically we are already in a multipolar world economy. The EU has the largest economy in the world. Japan is still a major player and even if China's economy is bigger than the US by 2027 (Goldman Sachs estimate) this will not mean that China is economically more powerful than the US. Mainly due to the unevenness of Chinese development and the huge lead the US has in the most important industries like technology and biotechnology. Transnationally problems such as climate change, terrorism and pandemics can affect every state across national borders, not only the US, more cooperation is needed.
One other thing we have to be more aware of than ever before now is the diffusion of power from states to groups, organisations and individuals. The massive reduction in cost of computing power means that it is difficult to know exactly where threats come from. Again this is a problem for every state not just the US.
A very interesting talk. And afterwards there are some very interesting answers to some questions too.