Are some mild involuntary head and mouth movements normal in elderly, (89) or are they only caused by medications like Seroquel (quetiapine)?

Involuntary movement. Are likely extrapyramidal symptoms from effects anti psychotic medications. Have psychiatric / movement disorder specialist evaluation as other CNs conditions can lead to involuntary movement.
No. Sounds like tardive dyskinesia to me. It could get worse. In my experience the benefits of the offending drug have to be WAY greater than the risks to consider continuing it. You need to confer with the doctor who is prescribing the drug & go over all of the risks & benefits of Seroquel (quetiapine) which also include metabolic, cardiovascular & an epidemiological increased risk of death in elderly users.