The mercury engine is in fact vary old technology that we have lost. You can find carvings of them in stone in India. A part of the age we lost. but then you have to be open to previous cultures being as technologically endowed as we are today and maybe more advanced. There are also some anti grave units developed just around the end of WW2 that work on electrical principals that were vary successful but suppressed.

On the point of getting into space.

Isaac Arthur on YouTube is a futurist that has some vary good insights into the issue. They are only based on current available technologies. I don't think he gets into anti grave.

In later videos he gets into elevators and space hooks Multiple space hooks look vary interesting. I watch his videos because I like his style and the depth of his cometary. For the most part it is interesting and understandable even he gets into quantum stuff.

Methane as a rocket fuel came from a spacex video were Elon mentioned it in his quest to make it mars.

As for Newtons laws they are just laws based on understanding of how things work based on a rather gross way of looking at things. Not necessarily wrong but defiantly limiting. As we learn more about quantum physics the more we will find the old laws will be replaced by new ones. Eer old ways of thinking vs new ones. The old ones give us a temporary step to stand on till we find a new foot hold. This based on another 'law' base on new observations and thinking.