I have had this question only answered to a degree and not fully. Simply, Park University needs to know if the data coming in through the Canvas App (or any other device) is safely landing within the Canvas Data, which means the resulting data (submissions, discussions, conversations, page views) can all be seen 100% [within reason] no matter how any activity enters in through the Canvas entry points (desktop, phone, iPad, Android, etc.).
I hope someone can verify both of these things, who either works for Instructure or has their expertise glued to the ground on this. I understand sometimes data gets lost, so nothing's perfect. This isn't that sort of question.
I'm sorry you have only heard a part of an answer rather than a full answer. Hopefully I can fill in the gap for you.
1. As for your first question. Yes you can absolutely guarantee 100% of your data is in canvas data. There should never be any missing data. This is because Canvas Data runs a complete dump of your schools Database Shard (each database is split into shard), every night. This is where the data is populated from. From the time it's dumped to the it will be the exact data inside of the database that the Canvas API/WebApp use. If you see any duplicated data, or missing data you should report it so we can fix whatever is causing it, and backfill the data (if needed).
2. Since the data comes from the data dumps you'll be able to retrieve them if you know the right api calls. The data could have changed since your data dump (seeing as how the data for now is 24-36 hours out of date (we're working on this)). However yes the data can be seen in the api.
Hey @rob_callicotte , Eric has given you some great information here. For now I'm going to mark this question as assumed answered. You can return and mark an answer correct if you feel it answered your question or provide more information if you would like further assistance.