My thoughts on the 5 questions are the following:
Defined, well known events that have their own tags should not be date tagged similar to History.SEHistory.SE. That said we need to be careful to ensure we do not up with tons of micro-events (i.e. 1980-Olympics) and keep the use of event tags to broad population impacting events.
If the asker knows both century and decade, they should mark it as both as long as it is not also tagged with a major event.
If the asker knows a general timeframe like the late 1800s they should not tag it 1870s, 1880s, 1890s; they should limit it to two decades at most otherwise just use the century tag like 19th-century
I believe it should be limited to three, even if it is two decades spanning like the 1890s and 1900s. It should only be a maximum of three with a century tag maybe being applied, not both centuries.
I think we should get retag most of the questions tagged with the date tag and then merge the tag into another tag. double-date questions I think should be re-tagged with the appropriate time period for the question and it merged into another tag. calendar also seem redundant being it is for specific timeframes for particular groups, but if we were to not merge one of the three I would keep this one.