In case anyone wondered what the outcome of all your useful advice was:
I purchased Paris/Basel return tickets via the TGV Europe website (rather than the Rail Europe website) and paid 50 euros (per person), instead of Rail Europe's £57. So that, combined with £69 Eurostar returns, resulted in better prices and a far better sellection of trains for both legs. Result is we get the desired 5 hours in Paris for a bit of culture and food.
Thanks for the useful input and information.
