[RPG] What constitutes cannibalism in Tomb of Annihilation

One way to do this mechanically is via the Help action

From "working together" in the Basic Rules (p. 59) you can apply advantage without resorting to anything "custom" to the adventure.

Working Together

The character who’s leading the effort—or the one with the highest ability modifier—can make an ability check with advantage, reflecting the help provided by the other characters.

A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another

One way to tailor this to the published adventure is that whomever has the highest Wisdom modifier tries to navigate, and the guide helps. Or, because they are not familiar with Chult, you make a group check (basic rules, p. 59) with advantage because the guide helps.

• You could make a ruling that unless the Wisdom(Survival) attempt, or help, is made by a character with proficiency in (Survival), but that's somewhat beyond the general theme behind ability checks: you can try anything, you'll have more chances of success if you are proficient.

You only need to roll the ability check if the outcome is in doubt.

Caveat: most of Chult is unexplored and poorly mapped. Most of the time, you can expect there to be some uncertainty, as described on pages 38 and 39 of the book, sections on the Land of Chult and Navigation. There are some NPC's whose back story includes being very familiar with a particular area, however, in the interest of avoiding spoilers, I'll get no more specific than that.

Guidance versus Guides

If the party has a Cleric or Duid, the guidance cantrip is a handy way to increase the chance that the party Navigator, be it Guide or PC, has an increased chance to not get lost/head in the right direction.

Starting from level 1 and running the full wilderness survival hex crawl, it's a full length campaign, probably taking anywhere from 6 months to over a year for most groups. You can shorten it by starting at level 9 and going straight to Omu, focusing on the events there and the tomb itself, which could probably be done in a shorter time frame. This part of the adventure is very heavy on puzzles and riddles though (each of the six floors of the final dungeon could easily take up a 4-hour session on its own as the party figures out its particular tricks) so it's hard to really rush it.