Step-in in case of misadventure or removal
A number of options could be considered.
One option is to have either the Retiring-President or President-in-waiting step into the role. This is probably not a very good option.
Another option is to have the State Governor of the state from where the President was elected step into the role. This is a better option because a State Governor would definitely be able to perform well in the role as President or Presidential-support. The State Government would then have to fill the role of State Governor with someone else.
In the cases where the President needing to be replaced was elected from the territories, the Governor-General could step into the role of President and the Prime Minister could then appoint another person to fill the role of Governor-General.
This kind of situation would only occur very rarely, yet this approach could be iterated even a number of times without affecting the schedule of elections. The terms in office for any one person are very short in any case.