Naturally I would love to see this letter and "All of the threatening language and accusations"
You could pursue your web designer, but that may be costly. I'll make a few points to your post.
1. "DeBoer asserts that went to the clients' website, removed the copyright information, altered it" he can assert all he wants, he would need to PROVE this IF it were to go to trial.
2. "There was no prior cease and desist"....none is required
3. "she had done the appropriate licensing searches for "free photos for commercial use with modification" ...I've never heard of such a search, and highly doubt there would be any results..there are very few reputible site offering "FREE" images for commercial use.
4. "I called DeBoer's office and responded to the email twice on the same day I removed the photo and haven't received a single response."...have no fear, you'll hear from them again, as they now know they have a "live one"
5. He also mailed printed versions of the attachments to my home. How did he get my mailing address when I don't have it on the website? .... a simple search of the domain whois database probably gave him this info, or a myriad of other places, it's very simple these days.
6. "Furthermore he asserts that the photo has been used for three years (since 2012)." again they can assert all they want all day long, that's different from proving something.
What is "LOC"

you should search for this image in the copyright.gov database, it may not even be registered, or it may not be properly registered, which would greatly reduce any award they would get IF it went to trial.