Herd immunity" is a euphemism.
No, it is an effect that is seen in longitudinal epidemiological studies. As the population that can be further infected diminishes the opportunity for transmission reduces and the case rate therefore drops off. At some point general transmission ceases, but reservoirs may remain.
What those who propose it as a strategy really mean is survival of the fittest.
Maybe, maybe not. CFR will tend to increase as societies ability to mitigate the illness is overtaxed. Countervailing issue is slowing rates by drawing out the timeline has consequences that may also result in loss from other matters; depression/violence/suicide/health effects associated with increased poverty, etc. Mutation is not directly a time effect, it is related to the cycles of infection that occur, the probability of a replication error, and the probability that the error is viable.
Disciplied personal hygiene remains the most effective infection control measure, which includes reducing contact opportunities. Vaccination will alter transmission rates, but to be globally significant the combination of resiatance (# vaccination x vaccine efficacy) + hygiene measures (contact reduction, PPE, fomite control etc) has to impact somewhere between 75 and 85% of the population. That includes all population, children, teenagers, youngbadukts etc, not just the aged, infirm or those with co-morbidities.
CFRs continue to be underreported due to methodology, the cases that result in current fatalities are those from 10 to 30 days before, not todays number of total cases. That suggeats that the true CFR in the cold light of day is around 3 to 4 times higher than the reported figure, enough to be a problem. Counter to that, the total cases are much higher than the detected cases, so, the realnrate will be determined in future post event analysis, and that could be higher or lower tham current reported cases. Early on, the undetected cases were around 20 x reported cases, they dropped off to around 10 x innthe last Q of 2020, now... Probably lower again.
Sucks