Many of the points you raise are actually generic to all jobs, not specific to airlines. It isn't discriminatory for a Western airline to favour hiring from their own country (or in the EU, within the EU). Every country in the world imposes restrictions on foreign nationals working.
So, the airline won't hold recruitment days in Africa if they can find enough high quality candidates closer to home. You wouldn't expect, for example, Goldman Sachs to go hiring in The Gambia when it receives 100+ applicants in the US for every position it advertises.
Countries issue visas, not airlines. If the airline wants to employ a pilot in the United Kingdom it can't just offer that job to anyone - the person must have the right to work in the UK. Unless the job is on the UK Shortage Occupation List - and pilots aren't - you won't get a visa.
Age limits are a matter of practicality and longevity. It is a fact that it is harder and slower to learn new skills as you age. The older you are, the less likely the training is to be successful.