The link you sent of passenger lists of ships arriving in SA is amazing - so detailed - but as you say the 1850 entry for the 'Jane' poses a few questions. There was no James Page listed among the passengers- though I suspect that the list is incomplete - so few names - maybe only cabin passengers were listed. Interestingly, though, there is a Thomas Page listed, and my ancestor had a son called Thomas, listed in 1841 as a grocer, aged 15 (the same age as James was supposed to be in 1841 according to my earlier unattributed source. Further, I found a personal ad in the South Australian Advertiser (21 June 1861) in which a James Page of Port Adelaide asks for information regarding the present address of one Thomas Page. The plot, as they say, thickens!
Ian