SmoothCruise,
In answer to your questions:-
1) You could say "There is space available to the right of the red Cessna"
2) The Air/Ground phraseology in CAP413 simply provides the pilot with information and is not an instruction to do/not do something. From the information provided it is up to the pilot to decide what to do. If the PA28 is on final it could be nearly four miles from the threshold and therefore it could be safe to cross the runway before it lands, however if has turned onto final from a conventional base leg it is likely to be safer to wait until it lands. The Air/Ground operator has simply provided the pilot with the information to make that decision.
3) The minimum age for the issue of an Aeronautical Radio Station Operator’s Certificate of Competence is 18 years.