The original Star Trek didn't have an "absolute speed limit of the universe", unlike Star Trek: The Next Generation. In fact, in the Star Trek episode, "By Any Other Name", the Enterprise is traveling at Warp 11. In "That Which Survives", the Enterprise accelerates out of control, ending up at Warp 13.9 before they do finally get the runaway reaction under control. Subspace communications travel at Warp 30.
The mathmatics really are not that tough to do. I routinely do cubes of warp factors in my head. Twenty seconds with a calculator of any kind and I can figure out any warp factor, including fractional ones. To me, the mathmatics are old hat.
I encourage you to disregard what you read in the Star Trek Next Generation Technical Manual. Why? For three very good (to my mind) reasons:
1. The scale in Star Trek: The Next Generation and subsequent programs is actually
faster than the scale presented in the original series. For example, Warp 3 in the scale used in the original series is 27xC. Warp 3 in the Next Generation scale is 39xC. So which is faster? The Next Generation's Warp 3.
2. The original series had no need of an "absolute speed limit of the universe", unlike the Next Generation. I don't like the "absolute speed limit of the universe" thing here for one very important reason: the absolute speed limit of the universe has been set at 1xC, and the warping of space to achieve faster than light speeds in fact skirts around the "absolute speed limit of the universe" by actually travelling slower than light within the bubble.
3. The warp scale as shown in the Next Generation Technical Manual is a solution looking for a problem, especially given the fact that the original series' Warp speeds are uniformly
slower than The Next Generation until you reach Warp 8 (new scale).
For more detail, look at
this page.
Switches gears and focuses now on the fusion system for your drive.
I misunderstood your fusion idea, and can see where it can initiate the reaction. You may have lost me just a bit with the talk of positive and negative particles here, because ion drives actually toss out positive and negative particles (protons or electrons), but not both.
As to space combat, I follow Traveller rules on that one, so there is no combat in Warp, so I've no need to worry about that possibility.