As you indicate, £30 was an enormous amount of money for a private soldier, representing around two years pay after stoppages. A private in 1871 would get 1 shilling a day but from this he have deducted various small amounts to cover his food and things like barrack damages. If he kept out of trouble for two years he might earn an extra 1 penny (1d) per day as good conduct pay, rising to 2d after 5 years of good conduct ( I don't think he would have reached the 5 year point for good conduct!). By 1881 the basic pay had risen to 1/1d and good conduct pay remained the same. There was a system in which a small amount of his pay was withheld and kept as compulsory savings for when he left the Army. These savings did not earn interest but were intended to act as buffer against initially being unemployed once he left the Army. As Rowland was in the Army for perhaps 5-6 years he might have accumulated around £5 in savings through this compulsory scheme, but obviously during his absence and his period in prison he wouldn't have been paid at all.
I think it is more likely that his family would have had to find the money to buy him out.
The 1871 Army Worldwide Index was not a census taken at the time. It is based on research done by Roger Nixon using the Pay Musters held by TNA which I mentioned earlier. You can find more information on this Index
here on FindMyPast (no subscription required). A shorter but similar description appears on
Forces War Records. There are similar Indexes for 1841, 1851 and 1861.
As for establishing when Rowland enlisted, the best source would have been the pay and muster rolls for that period (say around 1869-1871), but as I mentioned I couldn't find the relevant muster rolls. However having done a new search I think they may have been listed under the 12th Dragoons (the 12th Lancers were previously known as the 12th Light Dragoons). If I am right then the relevant muster rolls are:
WO12/1076 for the years 1868-69
WO12/1077 for the years 1869-70
WO12/1078 for the years 1870-71
WO12/1078 for the years 1871-72
I appreciate that these references may be academic for you if you can't get to TNA at Kew, but may be helpful if you either hire a researcher or ask the TNA to carry out the research on your behalf. In much the same vein here is a reference to another series which may or may not be useful. It is for the so-called description books of the Cavalry Depot for the year 1870:
WO 25/3892. Again not digitised unfortunately, but if Rowland had enlisted in 1870 and he went to the Depot for his training, he is quite likely to be listed in the description books at the Depot. I believe the Cavalry Depot (for Lancers and Hussars) was then at Canterbury in Kent. However it is possible that in about 1870 all recruit training for the 12th Lancers was being carried out within the Regiment. Recruit training consisted of two parts: basic soldiering in which the man was taught foot drill and how to shoot, and continuation training in which he learnt to ride and how to look after his horse etc. I am not sure how long he would have been classed as recruit but it was possibly a minimum of two months.
The two entries for a Rowland White in the 1871 Worldwide Index is interesting. Knowing how the Index was compiled, double counting is certainly possible, but as Roger Nixon explains, some data was taken from musters as late as December 1871, so it may not help to pinpoint the exact date of his enlistment. Seeing images of the two entries on the actual muster rolls at TNA might help to clarify if the unnumbered recruit was the same person as the one who was (presumably later) located at Aldershot. Incidentally, the 12th Lancers had been located in Aldershot from at least September 1870 (I haven't checked any further back).
