The mountain between Bluebell and Konohana is actually part of the natural border between two different countriesThat would explain the cultural differences between villages. The tunnel could be a shared mine and/or trade route between the two countries. That could also explain the Harvest Goddess having seemingly disproportionate reaction to the inhabitants of the two towns fighting in the past: she could have gotten afraid that the conflict between the villages would escalate in a full-blown war between countries and decided that blocking off the tunnel was the lesser of two evils.
The reason Pierre comes to every cooking contest in town......is because of the constant quarreling between the mayors. It's kind of amusing. As their friendliness goes up, they eventually stop fighting, which in case could also be a reason for Pierre to continue coming.