Did they require you blueprints and plans be reviewed and made to conform to their standards?

You would arguably be better off buying a CD at a slightly lower yield with almost no risk, time, or stress requirements. 

Its great that the seller is will to carry a note in a seller 2nd as well, that will help reduce your require monthly payment with the right lenders.

A lender is required to establish the ATR of a borrower when it will be their primary residence.

Some areas allow commercially zoned properties to share one large septic system (think sandmound or gravel type system) or would they require an individual on-lot septic? 

Do not expect to keep them and be prepared to have to evict.If the jurisdiction this property is in requires anything in place in order to evict such as rental licenses, etc., get them in place before discussing an increase with the tenant because it's much easier to get inspections and repairs done when the tenant cooperates.

However, they require consistent maintenance to avoid becoming a liability.

You can put a mortgage into a "trust: without triggering a due on sale but again contacting your bank/lender is the best route to avoid any issues.Some banks/lenders or servicing companies change guidelines and even a trust may require other paperwork but the attorney usually facilitates that to be safe.  

However if you had done a warranty deed (or a special warranty deed if you still have a mortgage on it) instead, your LLC has a claim against you for the issue on the title, and you in turn can go back to your initial title insurance to fix it.Some title insurance will allow to transfer the coverage to an LLC that you own 100%, but not all of them do or they may require an additional fee.

So this requires some upfront planning.