The error you get seems more like a bug in the engine. Is there any chance you could create a simple repro for this we can look at?
Those old iOS devices only expose 8 texture slots instead of 16, and so that is likely causing the problem here if a shader uses more than 8 textures.
Note that both cube map and sphere map use a single slot only, so that should cause no difference.
Anyway we would like to do environment lighting using spheremaps, is there any workarounds ?
Not that I’m aware of, blurry reflections using spheremap are simply not supported, and it’s not a simple thing to add.
You can turn off the clustered lighting support in the Settings → Editor, that would save you 1-2 textures on those platforms, or try to use smaller number of textures on the materials.