The R2A has a nice, bright, clear viewfinderwith a crisp focusing patch. No problems with flare. It doesn't have the eye relief of the ZI, but it should be fine with any of your lenses except the 25. i would think that it would be a good choice as a back-up. A change would be that the uncluttered 35 mm frame on the ZI becomes a shared 35/90 on the R2A.
Have you considered the new R4A? It will have built in frame lines for your ZM 25/2.8, thereby serving as a nice complement to what you have. Its effective base length will be pretty short at about 19 mm - comparable to the CL. But that shouldn't be an issue for most of your lenses & the ZI can handle your faster lenses in critical situations, i.e. up close & wide open.
