1 - I believe there is an option to clear in RAK. However see point 3 below
2 - just write IN TRANSIT and have your flight details to hand. that bit is easy.
3 - that is super risky IMO. RAK gets busy, RAM are often a bit late, BA require you to check in at a desk, so generally that is not something I would be comfortable with. If everything runs smoothly you'll make it. One small problem and you won't. If you choose not to immigrate at CMN then I would say you have a 0% of making this as immigration at RAK can often take 30+mins just by itself.

Sorry to give bad news but for a funky itinerary like this the research would have been good before booking....