To be sure, this letter is grounded in reason, however, if you read Chapters 57-59, you will see that it also contains a forceful warning for those who “shall disobey the words spoken by Him through us”. The letter is an exercise of papal authority, or, at the very least, is written exactly like the more forceful latter-day papal exncyclicals are written.
Regarding the number of the apostles, obviously, it first contracted with betrayal of Judas and then expanded to include Mathias and Paul. Yet, the initial number 12 was related to the 12 tribes of Israel.
Why would he do that if he knew (or should have known) Israel would reject him? And why would he say he was sent only for the lost tribes of Israel? But, then, he also doesn't know the date of his return...hmmm.