Hypothetically, if Musa Al Kazhim (a.s) was the divine religious guide for Muslims and had knowledge of the unseen (as per Shia beliefs), then it makes perfect sense. He knew that `Abd Allah would die soon and that if he opposed him during his lifetime, it would have caused a great controversy and split in the Ummah (again).
This is a poor explanation, because it does not make up for the failings of Ja'far as-Sadiq. The majority of Shiis took Abdullah as the Imam after Ja'far as-Sadiq, hence, he did a poor job informing his followers as to who was the next Divinely Appointed Infallible Imam (DAII).
Did the 'ilm al-ghayb of Ja'far as-Sadiq fail? Didn't he know what would happen after he died? Maybe it was eventually made up - but it caused much fitna for the Shias.
Also - the reaction of the Shiis after the death of Ja'far as-Sadiq demonstrates what Ahlul Sunnah believe - that initially the Shiis were mainly a political group, who supported the Hashimis against Banu Ummayah, and then the 'Alawis against Banu 'Abbas.
I am a bit confused by what you mean here. If he and his father (Ja'far Al Sadiq) didn't believe in this, why do we have so many hadeeths attributed to them that proclaim it? Al Kafi, for example, is 2-3 times as large as Al Bukhari and the vast majority of narrations go back to Muhammad Al Baqir, Ja'far Al Sadiq, Musa Al Kazhim, and Ali Al Rida (asws). Is it even fathomable that every single one of these was forged? I don't think so, personally. Can you demonstrate that these 4 people did not believe they were divinely appointed Imams? There's tons of evidence in Shia books to support that they did believe in it and teach it.
There are tons of fabricated ahadith, both Sunni and Shi'i.
Imam al-Bukhari reportedly collected hundreds of thousands of narrations, and only included a small % in his Sahih.
So no, it is not un-imaginable that the Shii books are full of 1000s of fabricated narrations (when we know that the initial Shii hadith collectors had no standards with regards to 'ilm al-Rijaal).
Ja'far had to be absolutely secretive about the next Imam after him. Otherwise, the `Abbasids would have killed his successor as quickly as they could have. Do you know of what happened with Ismail b. Ja'far died? The Abbasids went through every possible precaution to make 100% sure that Ismail had died (because there was rumors circulating that he would be the next Imam, so they had a vested interest in making sure he was dead).
As for the other children of Ja'far Al Sadiq, I am not knowledgeable on. I only know of Ismail, Abdullah, and Musa.
Brother - this is such a ridiculous argument. I know that this is what Shiis claim, as I heard it often as a Shii. But let us think about it.
Why would Ja'far as-Sadiq need to be "secretive" about who the next Imam is? Is it really that hard for Banu 'Abbas to just kill all of his male children?
No, instead, a better tactic for the Imams would be to keep in total secret the very
existence of their children.
Look - the reason why Banu 'Abbas killed the "Imams" was not because they believed them to be DAII. No, they killed them because all the descendants of 'Ali (RA) were potential rivals to their political power - i.e. the 'Alawis.