Attacking a heavily fortified position containing likely-at-least-four S-rankers plus a bunch of jounin, without offensive runes as an alpha strike, when large fractions of our assault force are SCs and can't take more than 1 physical stress, seems to me to be pretty much suicide
I don't think we'd be attacking a heavily fortified location without runes. We'd have 3 days of planning with Oro, Tsunade and Naruto on how to win that fight, and if a direct assault against the fortified position has such low odds then we'd just decide on a different tactic.
Hazo could set up his RER assault regardless of being spotted, forcing Akatsuki to come to him or even wait to see what he's doing. Which is an actual possibility if they don't know about runes and would rather believe in their sealing defenses over Hazo's sealing assault.
Hazo could set up superchillers from high enough that Hidan couldn't detect us, effectively out-ranging the assault. Not as likely to kill anybody outright, but it would definitely take care of the defenses.
Hazo could do absolutely fuck all personally, and rely on the gigantic boss summons to use artillery range jutsu to flush out Akatsuki and force them onto our own prepared ground.
I don't know exactly what the strategy would be, atm, but between 3 S-rankers, 6+ summoners, the planning skills of the Nara and the rest of the combined might of leaf, (+Noburi!) I don't think our chances in an assault are that bad. This was Orochimaru's first plan, after all, and Mr. Immortal wouldn't sign off on it if he thought it was suicide.
The RER ambush is better if we can pull it off, yeah. It makes sense Oro would swap to the new idea; but he's not the one getting bioseals implanted in him by somebody else with logical reasons to want him dead. On the contrary, as the one getting to put bioseals in somebody else, he might have additional, non-combat incentives to favor this new plan over the other that has little to do with the ultimate odds of success.