Hi whoeverishidingbehindPanda21andseemstoknowme,
Those are quite a lot of questions and to answer them fully would take a lot of time which at the moment I do not have. Also very few people will read this answer and my main reason to post on Steemit is to spread knowledge. So it would be better to devote a post in my blog on the issues you mention.
Fortunately I have already covered a few...

• I think I will do a post on lightning later, so I will skip that question for now.
• I know ElectroBoom and generally like his videos. Unfortunately he is ill-informed on Tesla and so he spreads poor info on him. He certainly did not generate millions of volts in his home. A few tens of thousands at most. :) I have commented on some of his videos.... :)
• In this post you can read about my idea about Tesla's theory of gravity. And yes, Tesla proposed to use magnifying transmitters to supply power to flying vehicles.
• The earth is a conductor, and since Tesla showed that you can transmit power over only one wire, you might as will use the earth as that wire. In this post you can read more about it.

I hope that covers your most urgent questions :)
Always happy to see some enthusiasm!