Maybe we have different ideas of what legal brilliance is. I thought that letter was at least 4 pages too long and excessively pedantic for what it is trying to say. The letter also spends too much time reaching conclusions rather than actually applying the law. Overall, it was about the same quality (from a legal and linguistic viewpoint) as Mr. Carreon's initial letter.
Moreoever, as to Mr. Carreon: Mr. Carreon has managed to make himself look like an expert on internet law to millions of potential clients through multiple media appearances. Mr. Carreon does not work for techies; he works for people who still think computers are magical black boxes. He's almost certainly okay with techies thinking that he is an absolute idiot.
Not only that, but Mr. Carreon has managed to make his client look sympathetic in front of the same audience (i.e., any potential jurors if the case goes to trial). Techies will find the drawing of the mom seducing the bear harmless (in part because it is, relative to the stuff we see coming out of reddit and 4chan on a daily basis), but ordinary people will look at that drawing and be horribly offended.
As to the IndieGoGo interference: by raising that issue, Mr. Carreon has suggested that IndieGoGo's TOS is toothless. It places IGG in a bind: if the Oatmeal did violate the TOS, then IGG must take down the fundraiser or lose the ability to use its TOS as a sword/shield in future litigation.
Mr. Carreon's job is not to look good, it is to represent his client in potential litigation. From a litigation standpoint, he's definitely got the upper hand so far.
(I am not commenting on the merits of his client's case, because I simply do not care enough about either party based on what is currently known about them to put in the effort to analyze the current or potential claims and outcomes.)