Identified and anonymous counts don't add up: They count different units — people versus browsers. See Why identified and anonymous counts don't add up.
Old rows have no pages, device or country: Per-person facets are computed from a recent window of events. Over a long range on a busy site, older rows fall outside it. Narrow the date range.
Country is empty: Country comes from edge request headers. Running against a local development environment, those headers are absent and the column is blank for everyone.
A contact has no activity at all: Either they were imported and have never visited, or their events have aged out under your retention setting. Retention deletes events but keeps the person.
Anonymous visitors stop appearing partway through the month: The month's pageviews are used. Anonymous visitors are not recorded again until the month resets; people you have identified are still tracked. Settings → Usage shows where you stand.

The Numbers Look Wrong