It is impossible to answer without doing an inspection of the crawl space. Every crawl space is unique and we recommend you to let professionals do an investigation before taking action. Your house is too valuable for chances.
As in the last question, it depends on how the ground looks and how far the mould attack has gone. One thing that is certain is that in most cases it is economically correct to fix the problems instead of letting them get worse.
No, because moisture damage can occur in many different ways, a dehumidifier is not a guarantee. But you are protected from moisture damages caused by the moisture that the outdoor air brings into the crawl space. Given that the dehumidifier is properly sized and installed. Make sure you get qualified professional help.
Do you have a question that is not answered here? Contact DST / your nearest DST representative that you will find on our contact page. Prices are only provided by the DST representative or a contractor.