I disagree with several of your points.
Scipio Aemilianus's prestige and influence had been waning since 137/136. At the precise moment when the great alliance of reformers was concluded between the Claudii, the Fulvii, the Licinii Crassi, the Mucii Scaevolae and the Sempronii Gracchi, and other less prominent aristocrats. The agrarian legislation was a mere consequence of this great alliance which won most major elections from 136 to 133 and 131 to 130.
I don't think Tiberius was uncompromising because of the Spanish humiliation. His election as tribune was the sign of the redeem of the population. If he was uncompromising, it was because of his character and extremely prestigious lineage on both paternal and maternal side.
And as you said he was but a member of an alliance, and a rather junior one, although highly vocal and prestigious.
There were a lot of unpredictable hazards in the crisis. Tiberius and his allies had not guessed that his friend Octavius would betray him nor that Octavius would for the first time use the veto against a measure so widely supported by the people.
And Scipio Nasica did not intend to murder his young cousin but just to oust him and his supporters away from the forum.
There were other cases of violent opposition between magistrates and tribunes in the decade preceding 133, the most famous with a Servilius Caepio.
Tiberius could have backed down without your WI, as I said in my previous post. Many others had before him, many others did after him. This did not prevent from having a brilliant career afterwards although others had a poor or even disgraceful military record (Publius Clodius for example).
Tiberius was basically consul from the cradle. Promoting agrarian reform and finally compromising would have guaranteed him further political success.
I don't see why you would state that Scipio Aemilianus' prestige waned before the Numantine War, when it would seem like he was at the height of his power when he was given dispensation to become consul so soon after his last consulship to widespread acclaim. He was very demonstrably able to assert his dominance in the political scene when everyone turned to him to solve the humiliating problems caused by the Numantine War. I agree with you that the alliance of reformers were developing at this point, but it wouldn't be until after Scipio Aemilianus left that they were able to act agains this interests. It is very telling that Tiberius is elected tribune and puts forward the Lex Agraria in the gap where Scipio Aemilianus was outside Rome and unable to act politically. The agrarian legislation was at the heart of the developing alliance of reformers, but the wider plank of reforms only really came into being under Gaius Gracchus, whose reforms in turn became the main planks of future Populare politicians.
Tiberius was hailed for his peace treaty in Spain by the plebs, but was attacked by the senate for his actions there. His election as tribune wasn't so much a sign of his redemption by the population, as him building on the support which was built in Spain. The treaty in Spain prevented him from running for the Aedileship, but left him popular with the people - thus he became a tribune of the people.
When he became tribune he went from being a junior member of the alliance to being the figurehead for the entire alliance as a whole.
I really think that you are giving Scipio Nasica far more credit than is his due, if he wanted to simply oust Tiberius' supporters he wouldn't have killed more than 300 people and then followed it up with a systematic purge of numerous Gracchan supporters.
I am not sure what cases of violence you are refering to, could you specify which Servilius Caepio you are refering to and other examples? Sources would be fantastic as well.
I am not saying he couldn't potentially have rebuilt his political career, but backing down would have been a major blow to his only remaining pillar of support. Also, keep in mind that he was the first to actually try to pass a land reform through the assembly, which meant that he had to create a lot of the precedenses which later politicians could build on. He was paving the path which later politicians would follow, and differentiate their careers from. Tiberius didn't really have anyone to base his actions on.
I just watched the Extra History series on the Gracchus brothers - fascinating men, who I'd shamefully never heard of previously.
I would really recommend reading up on them, they are very interesting. I am currently reading Mike Duncan's book on the period, which was where this WI started.
I, too, tend to think the differences would be greatest for T Gracchus himself. Class conflicts would have found other outlets and moments of escalation.
The conflict over economic inequality was definitely coming, but from everything I have seen T Graccus was being paned as the next "First Man" of Rome before the fiasco with the Numantines. The Lex Agraria wasn't Tiberius' favorite cause, but rather something that he supported in an effort to return to political relevance. If he doesn't have some sort of black mark on his career to work around, there really isn't anything preventing him from becoming exactly what people thought he would become.
This WI is basically focused on two points with a third logically following 1) what effects would Tiberius have in his further career on Rome, 2) Without Tiberius to spearhead the Lex Agraria - who takes up that role and how do their actions differentiate from Tiberius, and 3) Without Tiberius having established the Agrarian reforms and Populare approach as the focus of the Gracchans, what would Gaius Gracchus do instead?
Gaius Gracchus built a lot on his martyred brother, using him as a basis on which to build his supporting framework. With Tiberius not having died for a popular cause, would Gaius still have developed a Populare platform?