Yes - full marks to Blacksheep from a former physics and electronics lecturer.
Another little note on the second condition:
Batt 1 pos --> Batt 2 neg
Batt 2 pos --> Batt 1 neg
This is a short circuit - although the batteries are rated at 12v 30Ah - (They can supply 30A at 12V for an hour) the cranking power of the battery will be much higher (the current the battery can supply for a short duration). This could well be in the order of 300A here.
In a short circuit a phenominal amount of current will flow which will cause things to get extremely hot - you might even have an explosion on your hands, quicker than you can say "Oh &%$$+~"!
