A) You may be better off in the Medical forum. What exactly is it you have been asked to get? Is it to sit the RT practical exam? A FI is not qualified to say what is and what isn't a stammer nor what impact it would have but is only qualified to perform the RT tests. If this it then there a pre some good documents online.
B) Have a good look online, buy a book or get CAP413 from the CAA website.
C) There is a lot of air to ground coms particularly during take off and landing, in the TMA and if you fly anywhere East then you'll be switching frequencies and checking in every five minutes. More transmissions are instigated by ATC a compared to the pilots but regardless you need to be able to make and reply to all.