Update: I seemingly successfully replaced the backflow valve. Yes, I apparently need to have an annual test from a backflow contractor who sends the report to the city, about $30/yr, or else they will send me letter telling me to do so under threat of water shut off.
The shutoff value to the sprinkler system right next to it was also leaking that I wasn't aware of, so once I dug up the box it was in I then knew I had to replace that as well, nice brass right angle gate valve ($20), but was luckily informed that my local piece of crap big box store didn't carry it, but a specialty store across the street did. Thankfully it was before noon as well on the Saturday I did this so I didn't have to wait another weekend to perform the replacement.
Major home improvement lesson: shop well in advance for every possible item you might need, and have it all physically onsite before you start working.
I did have the Wilkins 350xl delivered to my house via internet big box order where I used a 10% off coupon I found online too ($110 delivered).
The new inspection report has been sent to the city, and so far it's been a month and no word from the city though apparently this work does require a permit and inspection to the tune of at least $80 and the indignity of not only asking permission to repair your own house, but subjecting yourself to govt standards and inspection (haha, like they have any...).
As long as the letters have ceased, I will claim victory and be happy escaping this ordeal with only about $150 out of pocket instead of the $1,200+ contractor quote I received. Oh and my sprinkler system works fine.
Thank you all for your encouragement, I leave you with words that can better describe my emotions (thank you Sara):
Everybody's been there,
Everybody's been stared down by the enemy
Fallen for the fear
And done some disappearing,
Bow down to the mighty
Don't run, just stop holding your tongue
Maybe there's a way out of the cage where you live
Maybe one of these days you can let the light in
Show me how big your brave is