Hi Geoff, 

Re: "It doesn't seem remotely plausible to suggest that they had access to
plenty of foods at all times, given inevitable winter shortages etc.,"

By that rationale you must find the survival of the Inuit implausible as
well. Yet they thrived through very long winters for millenia. Europeans
lived on Greenland for several hundred years but could not survive through
conditions that the Inuit not only survived but probably did so in relative
comfort. (See any of V. Stefansson's books)
 
You seem to have little respect for pre-agricultural human ingenuity. Yet we
had multiplied and populated the globe long before any sign that grain
agriculture had begun. 

Re: "if palaeo peoples really did have access to plentiful supplies of food,
why then didn't they expand vastly in population during that era if they
were less prone to disease etc. than in Neolithic times ?    The only
plausible explanation for the lack of a 
rise in population is that they practised mass infanticide as a routine
method to cut off population growth so as  to get round the limited supply
of available foods." 

Their populations grew so rapidly that they expanded throughout the world,
populating every continent but Antarctica. (See S. Oppenheimer's "The Real
Eve)

 
Best Wishes, 
Ron