OfflineSurveys supports having a "Previous" button since version 1.20 (released 2016-08-28). It is a little hidden though. Some background information:
When caching the survey, the all-in-one mode is required, but at that survey mode there actually is no "Previous" button (that wouldn't make sense). For that reason, we can't make use of the common previous button (it simply doesn't exist when caching).
So to go back to a previous page when running a survey with OfflineSurveys, simply press the device back button down left at the very bottom. It should then show you the app's "Previous" button feature.