I haven't tested the scenarios you describe extensively, but I think you may be overestimating the influence/contribution of GI in many of those scenarios. When you describe "objects that are naturally obscured over time by moving shadows", most of that effect is going to come from primary illumination, so the GI cache should not cause problems. With water, GI may very well not have a significant affect in many cases.
Also my understanding is that the way you describe the interpolation is actually the opposite of what you'd want to do to get flicker-free, smooth results. The more gi solutions you interpolate between (and, to a certain point, the *further apart those solutions are in the sequence*, e.g. every 3rd or 5th frame), the more averaged your GI solution will be, resulting in lower flicker. Yes, blending more frames reduces absolute accuracy and detail somewhat, but if your base GI solutions have a reasonably high initial detail (and you can benefit from no longer having to use high blur radius), then it should still look quite good, and generally preferable to fill light setups, etc.
So I would recommend at least trying GI caching in all the scenarios you describe. If you like the results of GI in your scene, flicker is very likely to be a concern unless you make effective use of the GI cache feature.
- Oshyan
