@Oeconomist
To quote "The American Heritage Dictionary" you suggested, on the definition of "shall": https://ahdictionary.com/word/search.html?q=shall
"The use of will in the first person and of shall in the second and third may express determination, promise, obligation, or permission, depending on the context."
the express of determination and promise is what is meant to be shown to a higher degree in the secondary, "will" here used in Einstein's speech, which falls in line with the usage of "will" in the first person case in this sentence.
"In the first person, shall is used to indicate simple futurity"
It seems "shall" in first person is only used to express "futurity", meaning it is merely a pointer to the event to be happening in the future, without any underlying tones the person saying it is determined it is to happen. So to me, reading this definition, in the first person context, "will" expresses a higher level of determination than "shall"
But anyhow,
"Such, at least, are the traditional rules. The English and some traditionalists about usage are probably the only people who follow these rules and then not with perfect consistency."
These subtle rules in the words are, lack of a better phrase, "complications of the past". "Shall" and "will" are largely interchangeable in the grammatical sense these days, and we were merely using these two terms in what we understand the situation to Einstein was. I don't think a deep dive into the original, traditional usage of the words is needed here for a manga translation.