The confusion here is from the initial poor description of how a battery works.
A battery consists of three things: a positive electrode, a negative electrode, and an electrolyte in between. The electrodes are made of materials that strongly want to react with each other; they are kept apart by the electrolyte.
The electrolyte acts like a filter that blocks the flow of electrons, but allows ions (positively charged atoms from the electrodes) to pass through. If the battery is not connected to anything, the chemical force is pulling on the ions, trying to draw them across the electrolyte to complete the reaction, but this is balanced by the electrostatic force-- the voltage between the electrodes. Remember-- a voltage between two points means there is an electric field between those points which pushes charged particles in one direction.
When you add a wire between the ends of the batteries, electrons can pass through the wire, driven by the voltage. This reduces the electrostatic force, so ions can pass through the electrolyte. As the battery is discharged, ions move from one electrode to the other, and the chemical reaction proceeds until one of the electrodes is used up.
Thinking about two batteries next to each other, linked by one wire-- there is no voltage between the two batteries, so there is no force to drive electrons. In each battery, the electrostatic force balances the chemical force, and the battery stays at steady state.
(I kind of glossed over what it means for two materials to "want" to react with each other. Google "Gibbs free energy" for more details on that. You might also google "Nernst equation.")
First of all, 30A in a short circuit wire with 12V is not safe. Chances are the thing will go bang or get extremely hot before you even have the chance to touch it.
As for your actual question 'how is it possible to have high voltage and low current and low voltage and high current?'. That question you have sort of answered yourself. It's Ohm's Law. you have used the equations to work out the current in those batteries. Now, change the resistance in that circuit and you will see the voltage changes, hence that is how it is possible to have high voltage and low current, and also low voltage and high current.
As for what damage can these currents do to a human.... Well, this link HERE will tell you all you need to know about fatal currents and electrical safety. There is also a discussion on what damage can be done in a question asked on this site HERE if you wanted a bit more of a read!
Best Answer
It is safe to do, yes. The current limiting of your power supply will only output the maximum selected current. You can do this for testing, learning by doing is always a good thing.
The question is, what you hope to achieve by this action. You will certainly see, that at some point your cable/wire is going to get warm due to the current flow, when you increase the allowed current. But you won't be able to make precise measurements and there are already tables online, that give you the values you are looking for under basically all conditions you can think of.
Therefore it will be a nice and safe to do experiment, but it does not make sense to really try to measure new data =)