When inverted, should my ultra stick remain reletively level? If it wants to dive, besides triming/mixing, is there any way to defeat this?

Yes it should remain fairly level. You might have to hold a little down elevator, but not much. Mine will stay level inverted with very little down elevator put in. If you want you can move the CG back and this will help a bit, but if you don't want to do that and it doesn't require much correction then I would just let it go and manually hold it level.

Most planes inverted will require some down elevator to keep them level, i.e. they will sink with no input. The quantity of input required depends upon many factors including plane design, wing/tail incidence, thrust angle, speed, CG, etc. A trainer may require 1/2 stick to keep the plane level inverted... a pattern ship may require just a breath of stick. I've also seen some planes that would climb inverted becasue of thier setup.

If your plane requires a lot of down elevator, and you want to lessen the quantity of down elevator required, the easiest fix is to move the CG back, as was mentioned in the previous post.

Another very often overlooked aspect in trimming an airplane is to look at is your aileron's neutral position. You can bring them both up a degree or two, and you'll be suprised at the difference this can make to an airplanes inverted performance.