High voltage dielectric breakdown & layering insulators
HV is trickier than simple addition, but in your first question, when breakdown happens between A and B, due to the air becoming a conductor, B is now roughly at A's potential, which means that breakdown will also occur between B and C. 

To put it clearly, in your question, B is a stepping stone for the voltage.

Now for the second question,  PLEASE DO NOT DO THAT

and for two reasons :


The reasoning above
Your life is on the line, do not allow any doubt


To elaborate a bit, in your question, you are B, what do you think will happen when (i said "when", not "if") the insulator breaks down?

Regarding your circuit with capacitors as insulators, your reasoning is only theoretically valid. In the real world, the capacitors have parasitic parallel resistance whose value can vary wildly, which means that the voltage will not be shared evenly between them, which can lead to failure. 

As a conclusion, I do not know what you are trying to achieve, but please seek professional training and advice.