You're original grade is protected, and if the moderator does downgrade it you'll never even know. Re-grading can take any amount of time, my Geography took a couple of weeks, my extended essay has still not yet been confirmed. And lastly, they are rarely successfull.
The grading system for subjective subjects (mine were History, English and Geography) works on a level basis for certain areas. So for instance in English your use and assessment of language for a particular piece of work can be awarded the highest level of competence, 5, or the lowest, 1.
So essentially, no you do not have precise mark schemes, and no examiners are not required to just look for keywords or tick you as you jump through the hoops.
Subject such as chemistry, on the other hand, do require key words in full prose answers. Otherwise key words will simply demonstrate that you have a higher level of understanding of the subject.