You are not the first and will not be the last I'm sure. Your best bet is to speak to one of the many RN & AAC chaps and chapesses that have trodden this path before you as they will have undoubtedly faced and dealt with similar issues themselves. In fact I'm also pretty sure this has been covered on a thread or two in the past so might be worth a search.
I would also advise caution relying too much on pprune derived answers for specific questions regarding financial / career implications. Speak to the manning people in both services as they are the experts. Much will depend on what career structure you are on, how much time you have served etc etc. Also you need to check / be aware of what career structure the RAF are going to offer you. I seem to recall a number of AAC people grumbling on here that they had not checked the small print of the RAF offer and were being held in longer than they anticipated.
If the RAF wants you they may be able to adjust dates to suit your RN exit date but dont expect the RN to let you go early - they will probably stick to the 12 months from date of your PVR.
Regarding resettlement benefits etc. if you PVR you will be entitled to some of them - again depends on length of time served. You may even be able to start some of your RAF training whilst on RN terminal leave?!?!? although not sure where you would stand regarding insurance etc whilst on 'leave'. Speak to your unit resettlement guru - thats what he's there for!
Finally I would suggest that you dont air your dirty linen in public. Obviously dont know your personal circumstances and you may be in a grump with the RN cos they wont let you do what you want now but be careful of burning too many bridges and upsetting people before you have to. Otherwise if the RN are going to lose you but have 12 months to play first stand by for plenty of SLJs. You have of course anyway opened yourself up for plenty of abuse from the die hard fish heads roaming this site!
Let me know via PM if you need any more specific info.
SW