Nope, a market value opinion is not purely subjective - it is a valuation model based on certain principles that exist in the MV definition, to lead to a credibly supported "market value."
Prices are facts, values are an opinion -and the appraisal has a MV definition which sets out the terms for the oponin - one of which states MV us what a Typically motivated buyer would pay, and a typically motivated buyer is not an individual who pays more to be next door to their mother, which addresses your example. Thus, if you wanted to use that sale as a comp, you might reject it when you found out the reason for the higher price, or you might include it as a fourth or fifth comp and give it less consideration and explain why.
You are bringing up examples of individual buyer perspective, which is called value in use. If 90% of garages are used to park cars ( in my experience that is what I see, ) the fact that some covert it to a man cave doesn't; matter much - again, it is the actions of the typically motivated buyers for that type of property which matters in a MV purpose appraisal.
Litigation makes up a tiny percent of res appraisal practice and most appraisers never go to court or go once or twice in an entire career.
Value is not just something printed on the form. Value is the purpose of the appraisal, and the value is defined in the MV defitno !! If an appraiser says they can not define it because it means something different to each buyer and seller, they need a refresher class-
In life, Value can mean something different to each buyer and seller , but for an appraiser, we arrive at Market value by using the "model" of the typically motivated and well-informed or well-advised as set out in the MV definition.