Hi there – my daughter sat the Birmingham grammar test a year and a half ago. It was CEM then, but the principles are very similar, and if you are self-preparing, the choice of workbooks is much the same (you just buy the GL ones instead of the CEM ones).
We self-prepared our daughter too. We preferred, and trusted, the CGP books, but we did make sure she did some work from other providers, as it is easy to get used to one style of question and format in terms of the size of font, numbering, style of pictures etc. So, I would suggest using a few other option as well. We had one or two Bond books, and also Schofield & Sims, Letts, Collins.
We didn’t do work every day: maybe 30 minutes, 3-4 times per week – but everyone is different. We felt that was enough. We also didn’t work through every page of every book. If my daughter was finding a section/topic easy, we’d skip some or try to find something harder. If she found a section hard, we’d broaden her experience by offering a variety of material at a similar level. We didn’t worry about timings until about Easter, but we did make sure we went through any questions she got wrong, and we also asked her to put a mark by a question if it was a bit of a guess, so that, even if she got it right, we consolidated.
I don’t know about KEHS as we only tried the state grammar test, but for the state test I shouldn’t worry about others carrying around 11-12 books. Using 10-11 books and getting most right within the time and having a good approach to the questions and good exam technique should be enough. Our daughter passed very easily, and is now at the grammar of her choice, and we didn’t look at 11-12.
Regarding mock tests, I think it’s a little bit early. My daughter did a test in April or May with a small provider, and then she sat two ‘proper’ mocks (100+ kids) in June and August, primarily to get experience of the test conditions. The results from these tests were more helpful than those you have had: we received an overall percentage, plus a percentage for each section and each topic, and also the highest and lowest score for each vs our daughter’s score, and also her ‘position’ amongst those that sat the test that day (both overall and for each section.) This was really reassuring (eg when she had found a section hard and could then see that she wasn’t alone!)