I’ve thought that ‘on’ gives the nuance of continuity. So the difference between 1 and 2 would be the property, is the thought. But is there really the difference? Or are there any other differences? Or there are no differences at all?
I think they mean exactly the same thing.
Catch fire is by far the more common idiom; catch on fire seems to be due to influence from on fire.
There also seems to be a regional difference; in the British National Corpus, the ratio of caught on fire to caught fire is 3:144, whereas in COCA, the ratio is 88:542. So whereas caught on fire sounds fairly normal to me (an American), albeit somewhat colloquial, I'm guessing that most British speakers would find it noticeably strange.