Ill throw in my 2 cents worth here. I am an underwriter for a national ag property and casulaty insurance carrier. We dont do homeowners insurance, but it all works very similarly.
You should be able to choose from replacement cost, actual cash value, or stated amount. It sounds to me like you would like stated amount coverage. Insurance companies generally dont love to do stated amount, but you SHOULD be able to find someone to do that.
If you are covered for replacement cost as of now, it is important that you and your insurance company come up with what you think the house is worth and agree on it. Coinsurance will come into play if and when you have a claim. If you arent insured up to the value that your insurance company determines the property is worth, you will be only paid a portion of the claim. It is kind of complicated, but you will be paid roughly the same percentage of the claim as the percentage you are insured to for the property.
I definately see both your and the insurance companies point on this issue, but I do get that its frustrating.
Hard to explain, but make sure you get things straightened out or it could bite you in the a** if you had a claim.