From what I've seen when applying for courses (though saying that they haven't been engineering or economics) the number of GCSEs don't matter tooooo much, well its don't matter if you have 10 or 11. I got 9 GCSEs and met the GCSEs requirements at the courses I'm applying to. They aren't a huge decision factor, just make sure you meet them.
You don't need FM gcse to do FM a-level, and lots of people manage very well at the a-level without it. Don't force yourself to do it if you feel that it'll push your other grades down. If you can manage a bit longer,, it may be a good idea to see what your mock grades would be like and you can decide if they are affected by GCSE FM.