There are at least two posssinilities for dimensions, I am still
thinking about if we should have U1xU2 for space and time or
U1xU2xU3xU4.
As for urelements, they are "objects" which do not contain anything and which are undistinguishable, which fits space and time.
As for the lack of compatibiliity with other theories, it would be seen by critics.
May be you can help.
Adib Ben Jebara.