All i know is i would absolutely never install a direct connect for anyone regardless of what they say their intentions are.
Absolutely, and I see where you are coming from, as an installer. Quite a bit different than letting people dig their own hole to climb out, you are not in the business of helping them do things the wrong way! That puts you on the line for later problems, so I have all the respect for you not doing that.
Also agree 100% on that insulted liner posing a much lesser risk of house fire. I'm not really disputing your points, or suggesting you change your tune... just that the OP has options, and for a four-month window of occasional burning that doing it the wrong way is an option... potentially unsafe, but also potentially safe if due care is taken.