I think it's also about the order in which you apply the "rules" stated above:
1. I always start with going to the AB evaluation, and look for the OUV statement (in the newer documents) or the explanation of the Criteria why it was made a WHS. Sometimes this will already give you a year or a time-span. If so, then I stop looking further.
If not, I will go on searching using the hints that are given in the OUV / Criteria: for example: medieval trade town, Phoenician settlement, Bronze Age rock art.
2. I will search the AB evaluation for years or centuries that stand out and are linked to the OUV. So if a medieval trade town also has some Roman ruins, I stick with dating it to the Middle Ages.
Sometimes the OUV will point to two or more flourishing periods, this will result in attribution of multiple periods to a site. (only if they are not consecutive)
3. If there is a longer period, say 12th-16th centuries, I will take the earliest date (12th).
4. When it's still not clear to which period the WHS belongs, I use the dates of the construction of the major monuments (a cathedral, a Roman theatre etc) to determine its age. I then also start looking at other internet sources (wiki and others). But still I am not looking for the oldest structure on site, but the oldest major structure that can be linked to the OUV.
Furthermore, specific rules apply for cultural landscapes (I like to date them to the period when the practice stated in its OUV started, so when they started to grow vineyards on terraces etc). And I agree that the style of any major rebuilding decides if we date it to the original period or to the period of its reconstruction.