Odds are they found your images because you credited Larson. You could show and ask Larson for permission. Normally the take downs do require each individual item to be listed with it's link. Many times you get a notice and depending on the website host they receive the take down notice and decide whether to comply. This sounds like a para legal doing work trying to justify the lawyer's fees to the client. I'd suggest confirming via google that the attorney is real. Odds are he is but still....

As to your work, you could repost it without mentioning Larson and his comic line directly and state that you are inspired by a particular artist and these are homages to his work. You could also state fair use doctrine (google it to familiarize yourself) and satire as well as you not selling any work or making any money from said work. Odds are you're on the radar of that office and they'll check your posts at the different websites. Removing the work kind of in their eyes and interpretation admits to their complaint that you were possibly infringing. Sadly that's the way many attorneys think and operate :( If you really wanted to cause the attorney some grief you could send a letter to the EFF (Electronic Frontier Foundation) and see if they will give you advise or take your case free (as they do tend to fight for these kind of things).

There are several companies and artists who are really bad about abusing DCMA takedowns without any justified reason.

The dust settled, thinking "what a fine home, at least for now" not realizing that doom would soon be coming in the form of a vacuum cleaner.