The Rulebook, page 11: "If a contestant is absent from the briefing, the alternate speaker, if present, is permitted to attend the briefing in place of the primary contestant. If the primary contestant is not present when the person conducting the contest is introduced, the primary contestant is disqualified and the alternate officially becomes the contestant.
Should the primary contestant arrive after the briefing but before the person conducting the contest is introduced, the primary contestant is permitted to compete, provided the primary contestant:
a) Reports to the contest chair upon their arrival.
b) Has all required paperwork in good order.
c) Waives the opportunity of a briefing."
Which means: If a contestant is running late, they have until the Contest Toastmaster is introduced by the MC before they are disqualified. It is fine to have the second place-getter attend briefing just in case the contestant is too late, but if the contestant arrives within the requirements, they are still the primary contestant.
If a contestant (or a representative) is not present at the briefing, they are still included in the draw for speaker order; they are not defaulted to last speaker.