why would OPA spend a substantial sum on legal fees if it did not intend to "avail themselves of the decision?"

To show whose the boss.  To prove a point.  To flex their muscles.  Who knows.

Maybe the real reason, and only reason, is because the porch violated ECC edicts.

Regardless, just because they can doesn't mean they have to.

 I understand the legal costs were relatively substantial. Maybe more than the porch was worth.

One would have thought somebody in authority within OPA would have stepped in and tried to resolve this issue before the decision was made to spend such substantial legal fees.  What were the alternatives?

In such cases who makes the final decision as to whether to proceed with legal action that will result in legal costs more than the porch was worth.  Any checks and balances in place?  Is this a one person decision?