I would recommend you drop down to 3 subjects for A2 as most (all?) universities only require you to have 3 full A levels. This would give you more time to concentration on each subject.
I've heard that economics resits are easier once you have learnt the A2 content, though I'm not sure if it's the same for your exam board.
To get an A* you need 90% in the second year modules with 80% overall. So yes, with work you could still get an A* in either subject but it depends how you find the A2 content.
P.S. Well done on the French grade! A level French is a pain. :P