Carl,
with regard to your questions on eyesight, aptitudes, the RAF, RN and how many times you can take the apptitude tests, well I just don'tknow.
What I would suggest is that you write to both the RAF and the RN to ask them for yourself. Don't tell each that you have written to the other - an independent answer will be useful to you.
I believe (but can't promise) that the "2 goes only" rule means two goes PER SERVICE so you get 2 bashes at the RN and 2 at the RAF. However, if you have passed the RAF apptitude tests then the Navy may not even demand the full works from you (at least, they may let you off the piloty tests - they will still want to see you on their officer apptitude tests).
Again I can only advise that you write to them and then keep hold of the letters when you get the reply - the people you deal with on a particular day may not know what Joe Bloggs to you last month.
Good luck - if you get into any service in any aircrew role you will enjoy it.