Right now events in MobileWikiAppSessions schema (and other schemas) are unreliable due to the absence of client-side timestamps.
For example, a session summary event summarizing the previous session is sent at the start of a new session. This can be days after the previous session. Furthermore, if the device goes offline those events are put into a queue and are pushed out when the device is online and the user opens the app. The only timestamps we have are server-side from when the events are received. This means we can't reliably track session length metrics over time and it also creates additional challenges for T184089 (hence "High" priority).
According to @Dbrant the app has a record of session start time that it uses for checking when to start a new session, so it shouldn't be too difficult to add that info to the sent events. He also agreed that it would best to standardize & fix all the funnels together rather than just the sessions one.