2) The heir would be addressed according to the normal protocol i.e. crown prince/ss (or the equivalent title) XX
That would be definitely the case in Belgium where, under the constitution, the heir does not ascend the throne until he/she actually takes the constitutional oath before a joint session of parliament, which he/she can only do after turning 18. However, in the UK, my understanding is that the heir by law automatically becomes king/queen once the throne is vacant. In that case, shouldn't he/she be addressed as "His/Her Majesty King/Queen xxx" even if, due to his/her age, the royal prerogative is being actually exercised by a regent ? Maybe the regular British posters could clarify that.