With regards to the 1758 shillings and sixpences being available in some quantities it's because they were struck for a number of years, into the early part of George IIIs reign. I suspect even at this point though silver had been fairly scarce for a while. The larger silver coins had dried up earlier in the 1750s, I have read articles in the past that stated that even the great silver recoinage of 1696-8 period was not successful and that silver shortages were still somewhat problematic, even after weeding out the substandard hammered remnants - although not to the extent of George III's reign. I don't think much silver was coined (at least smaller silver - shillings & sixpences) in James II or William and Mary's reign - halfcrowns are more prominent. The basic problem being that the silver coin face values were undervalued - British silver coins were worth more melted down on the continent. The inverse is true of gold though, Britain had a plentiful supply of gold right through from Charles II to the Napoleonic period.