That's a very very interesting idea, the spool of thread, but it also poses certain problems... However, I find it interesting because it does not deviate significantly from the idea of a sphere. However the question is, what one earth is the center bit made of! In objects of relatively small(by small I mean human proportions), such a device is possible. However, when one gets large enough so that the gravitational constant G is no longer a significant barrier, one needs an incredibly strong material to prevent crumpling into a more stable object. What is this material? And remember, at one point, if there is a core, that area is going to be very weak due to the rather sad fact of liquid dynamics. About monopoles, too large means that you'll never see them, meaning that it would have to be on a cosmic scale, and they're properties would be easily observed. It would also mean that there would be no south pole.
Sadly, the monopole theory doesn't work(believe me, I wish it was true, they have lots of useful applications that would be cool if they existed). You're best hope is some sort of thing similar to how spherical Earth gets its field. GUT theories and TOE theories(yes, I know those are repeated phrases there, but it helps with clarification), predict their possibility, but if they exist there, why can't we create them, or at least dig some up? If there are monopoles there, we should be able to scale down into the spool's center, and extract some monopoles. That would help your theory a lot.
I have heard theories of dark matter interfering with gravity; sadly, that doesn't do anything. All dark matter would do is help the earth on its way to crumpling. Gravity, unlike magnetism, has only one charge. Nice discussion though, nonetheless.
Some people claim that scientists hate the flat earth theory. That's not exactly true. If flat earth theory really was true, there would be fantastic possibilities in the applications of such wonder forces. If dark matter or dark energy really was so close, it would be every physicists dream to grab some and study it. Occam's razor merely states that the simplest explanation is the best, and round earth is the simplest. Nonetheless, gedankenexperiments are fun, and I enjoy plaing with them.