Yes, the first one was one of the sources I thought about. Henning amongst others have used this article as a source. The Ningbo and a couple of the Ming dynasty texts are mentioned there. The author also mention another text where "neijia" is brought up, but I can't find any source for that text online so I don't know what year it's dated. Should be quite early though.
Here in this book, "Investigations into the Authenticity of the Chang San-Feng Ch’uan-Chi: The Complete Works of Chang San-feng," you can also find a lot of sources and quotes, some of them earlier than "The Epitath". The whole book is downloadable in PDF format:
https://openresearch-repository.anu.edu ... 389461.pdf But I don't remember if there is anything about boxing or neijia mentioned here. The book deals with texts that are supposed to have been written by Zhang Sanfeng, but the author's conclusion is that it's all about speculations and that there are no proofs that Sanfeng has written anything.
The obvious conclusion though is that Zhang Sanfeng was early associated with chinese martial arts, in various texts he is mentioned together with teachers, and students amongst other things. And several texts mention different names associated with Chinese martial arts. So regardless it's all facts or fiction, there is no doubt whatsoever about that the name "Zhang Sanfeng" has a connection to martial arts already in the early Ming dynasty. (in the 14th century) Frankly, I don't understand why people have hard to accept this or claim that it's a recent invention. Daoists practicing strange forms of martial arts was nothing new. Old Wuxia legends and common Chinese myths are filled with fighting daoist monks, and there are several well known historical people who are associated with "Quan" as well. Daoist philosopher Ge Hong (283-363) for instance wrote:
"All the martial arts have secret formulas to describe important techniques and have secret mysterious methods to overcome an opponent. If an opponent is kept unaware of these, then one could defeat him at will."
IMHO, this almost 2000 year old thought is the same as common principle in IMA today, to hide intention or as said in TJQ that you skills should "suddenly appear, suddenly disappear" or "from nothing to something, from something to nothing."
Some scholars trace the historical roots of Tai Chi Chuan and IMA far earlier than Chen Wangting. And they have very good reasons to do that because their argumentation is based on real historical facts, but their reasoning and the historical sources are very rarely brought up in modern discussions.