I don't know very much about the IB, but I know an alright amount about your law-related questions. Without claiming to be an expert, I would say that having a more 'traditional' essay-based subject definitely wouldn't go amiss - economics is sometimes regarded as being on the cusp of science and art, because it's so mathematical and technical at times, although I recognise there are strong essay-writing aspects to it. Maybe you would want to consider changing on of your science subjects to history or something? It's up to you - I doubt it will make a huge amount of difference. I have no idea about the seventh subject point, but I am fairly sure that they don't really care about whether or not you're doing 4 A Levels, so I can't see it being much different for IB.
As far as your concerns about law or concerned, I don't think you should be worried. A law degree is seen as a very useful qualification to have, since it is regarded as one of the more difficult of the arts subjects that you can choose to study. Typically the workload is quite heavy and the nature of the thinking required in law is seen as useful, even if you don't choose to go into law.
Your salary as a solicitor will vary heavily on the sort of law you want go into. If you choose to go into commercial law and work at a big City firm (which many do), you can expect to earn about £40,000 in your first year, rising to around £45,000 in your second year, and finally to over £60,000 on qualification (or, if you were to work for an American firm, as much as £100,000). Commercial law pays well, and if you were to become a partner at a law firm then you could comfortably be making several hundred thousand pounds a year, I believe. If you're looking at something more like family law or criminal law, you can probably shave about £20,000 off of each of those first three figures (unless you're doing it in a City-like context at a firm like Farrer & Co or Withers, where the salaries are still very good).
Hope you find this useful. Good luck!