Re mutation: I've read stuff to the effect of if there is a lot of virus it may mutate, yes, but the mutations won't be selected for, since the original strain is surviving. But if that strain is under pressure and is being neutralised by vaccine-induced antibodies, and mutated strains are more resistant to those antibodies, a new variant will survive and replicate until it becomes the dominant one. (It will be 'selected for').
It is basic evolutionary theory.
Mutations are random events which occur when the virus is being replicated in a host human - the more the level of the virus in the population the more mutation events will occur.
The mutations may, or may not, change the characteristic of the virus - specifically making it more or less transmissible than the virus before the mutation or more or less dangerous in terms of severity of disease than the virus before the mutation.
If the mutation is more transmissible it will be more likely to infect people and more likely to infect more people and therefore there will be greater overall replication of the new mutant and it will replicant and survive better and therefore become dominant. It its transmissibility is less than the pre-mutation variant it will do the reverse and will likely disappear fairly rapidly. If there is no difference in transmissibility than the original virus it will likely just transmit itself alongside the original.
The issue of severity of disease is different as it doesn't impact on replication unless it it so severe that it kills the host before replicating or is retained for much longer in the body before the immune response deals with it.
So in terms of a pandemic your worst nightmare is the combination of a virus mutation that is more transmissible and causes more severe disease. But your friend (up to a point) it a virus that is more transmissible because causes less serious disease, allowing transmission (and population level immunity to develop) while only causing very mild illness.
