If the value of a home drops, can the mortgage-holder accelerate the balance of the mortgage?

No, you have nothing to worry about.

Your mortgage is simply the money that you borrowed with an agreed upon payment schedule. The bank can't suddenly up and decide to not honor that agreement and make you do something else.

The bank isn't allowed to make the rules as they go along. They're only allowed to do what is specified in the mortgage and I've never seen a mortgage that allows the bank to accelerate the repayment period based on changes in the home's value. If you keep making your payments, the bank can't change anything.

Keep in mind that the assessment from the county tax office is usually not very accurate.

They can do whatever your mortgage contract says they can do, but I haven't seen any mortagages with provisions for requiring additional payments to get the balance below current value more quickly.

Of course, selling or refinancing your home will be nearly impossible until you owe less than the home is worth.

Also, keep in mind that assessed value is often much lower than real market value. If you are considering selling, don't let the county's assessment be your guide in what you could get.

The answer is no. The lender can't unilaterally change the terms of your financing contract any more than you can. The county's assessed value is not necessarily the MARKET value, by the way.

Your loan agreement is fine as it is. It cannot be accelerated by the bank.