my expectation is that al Sadr would go along with Khamenei
I think your expectation is belated as al-Sadr independently og the Iranians has already stated he would respond if Iran were to be attacked by the US. So it's not a consideration what Ayatollah Khamenei wants.
Also remember al-Sadr is not a religious scholar as was his father or al-Sadr's brothers. He has been for the most part a politician without religious credentials. (This may be changing but no one has verified those reports, last I heard).
Imputing that Ayatollah Khamenei could issue a fatwah which al-Sadr would immediately follow is akin to imputing that if the Pope told JFK to do something he would have. These guys can and do operate independently of their religious affiliations and in this case al-Sadr may consider nationalism far more important than religion, remembering his father dictum, governance of the people.
evidence of the tension between the two.
There is in fact more than just some evidence, it is a well known, and in fact al-Sadr called for the expulsion of al-Sistani for being too close to Iran. This suggests al-Sadr is not as close to Iran as some would have us believe and certainly not as close as SCRIRI, like al-Hakim.
al-Sistani and al-Sadr compliment each other and thus offer a complete package for Iraqi consideration when and if elections come.
Anyway by the end of the year we should have a better idea of what will happen.
