Even with things like mob griefing off, graves can easily be taken out by explosions. Would it be possible to add a config option to make them more explosion resistant?

It is already set to extremely high value. But It can't protect graves from destruction of ground under it. Are you sure that problem in graves?

If the reason of this probles is a result of ground destruction - try to set "OnlyOwnerCanLootGrave" to true - it should protect grave from destruction by any source except destruction by its owner