It is not really possible to give you advise on these structural matters without actually going into your house and looking around. The type of framing that you describe doesn't sound all that odd to me. Not used much these days though. Since I am not there I can't tell you what is holding it up, in fact, it may be inertia - as you suspect. But it might also be other things too. Does it have an effective ridge beam? You don't need collar ties or joists if there is a structural ridge beam holding the roof up - no pressure down, no spreading of house. Also, construction practices were different back then and there was less reliance on nails and more on jointery and pegs, etc. so the method of support might not be apparent. If you have doubts regarding the structural integrity you should have a home inspector look at it. Well worth the money. It sure is easier to fix before you put on the plaster and trim and THEN have something fail or need replacement that you didn't anticipate. Do you have powder post beetle damage in any structural members? I would want to know this if I were you.