A lot of that - the tonnage data - is way over my head! However, do consider all the circumstantial evidence:
I read somewhere (sorry, I don't have the source) that the changeover to bronze was originally to have happened in 1858, but there were so many problems with the thinner dies that it was pushed back. This makes sense if you then take into account all the very many varieties of 1858 penny - that they decided to use up existing dies resulting in several overdates. At greater leisure they produced dies for 1859 (very few varieties), which then got pressed into service again in 1860 when 30k pennies were needed in a hurry.
If the changeover didn't happen until late in 1860, that would reinforce the 'low mintage' rationale. Meanwhile, if they didn't experience the same problems with the two smaller denominations, they would have gone ahead with minting those bronzes. The 1860 copper halfpenny and farthing - much rarer - could indeed have been mementoes not intended for circulation.