1) Do you need to read the CFAI readings thoroughly for L3? No
Subject to the exception I make in (2) below, you don’t need to read the CFAI books to pass.
Schweser + CFAI EoC + couple of CFAI mocks was more than enough for all 3 levels.
2) Is Ethics really that difficult? From the perspective of a native English speaker, no.
When I started L1 a colleague charterholder told me Ethics was critical. Worried by his advice, I was extra thorough - read the CFAI text in detail and wrote detailed notes. Fortunately, this proved invaluable - barely had to do more than skim Ethics in L2 and L3 as a refresher.
Ethics is pound-for-pound the best topic to know well for the exam - know it well and you can fly through the Ethics item sets, banking plenty of time for the tougher questions later on.
Personally, I found Ethics the easiest topic on each exam (contrary to many on here).
3) Are mock exams helpful? Schweser mocks aren’t helpful - avoid. CFAI mocks invaluable, but I only did 2 before each exam.
4) Can you pass with less than 300 hours study? Yes.
I managed to pass with <250hrs for each level. Of course this varies person to person, but if you have a background in finance, work less than 60hrs a week, don’t have kids and are discplined enough to allocate, say, 8hrs each Saturday for 6 months (and 2-3 full days pre-exam), it can be done. Personally, it helps I had an understanding wife.
5) Tackling the AM: When I started each section of the AM, I wrote what time I needed to move onto the next question. Eg if I had an 18 minute question at 10.30am, I wrote 10.48am at the top of the page so I knew when to move on. Remember, you get partial credits in the AM - if unsure, at least write up the formula, take an educated guess, etc then quickly move on!