There is this old blog: Ahlul Bayt
If you click the links about witnesses, he proves well, that there is a witness in each time. Some of the translations I disagree with, I would translated ummatan in Ibrahim (a) prayer and the intermediate umma as "course/path" as we have hadiths interpreting it that way plus this is the emphasis else where such as Ibrahim (a) being an ummatan (which doesn't make sense to be nation). But it's good logical reflection over the verses about witnesses.
If you click the links about witnesses, he proves well, that there is a witness in each time. Some of the translations I disagree with, I would translated ummatan in Ibrahim (a) prayer and the intermediate umma as "course/path" as we have hadiths interpreting it that way plus this is the emphasis else where such as Ibrahim (a) being an ummatan (which doesn't make sense to be nation). But it's good logical reflection over the verses about witnesses.