Without Leo, I don't think season 2 Hürrem would have resorted to some of the more extreme measures that she does with Princess Isabella. After all, her approach to romantic rivals changed in season 1. She's way less brutal with Viktoria than with Gülnihal. So I can see her continuing to be more subtle than for example sending Isabella a snake (the first time in the franchise where she actively tries to murder someone).
Didn’t her attempt to poison Ibrahim with his notebook predate the snake? That presumably wouldn’t happen in a scenario without Leo’s death. Without Leo in the mix, Ibrahim’s worst acts against Hürrem were sending Gülnihal to halvet, which had no significant lasting consequences, and leveraging the proof of her innocence to force her to perform public penance, which she turned to her advantage. Nothing that she’d kill him over.
Without the poisoning, Sultanzade Mehmed would have been stillborn. Hürrem was only present at the delivery because she visited so Nilufer could plant the poisoned notebook. Would Suleiman have had Hatice host Isabella after the fire at the hunting lodge if she were grieving for her baby, or would Isabella have ended up at the Old Palace or somewhere else?
I agree that Hürrem is unlikely to have tried to kill Isabella if she didn’t already see herself as a murderer.
With the Russian concubine, Tanya, Hürrem hurt herself rather than her rival, and intended to take her own life if Suleiman didn’t send her away. Perhaps, instead of trying to kill Isabella, she would have attempted suicide. If so, I can see that shocking quite a few characters. Only Hürrem, Suleiman and Gülnihal knew that Hürrem threatened suicide over Tanya’s continued presence in the palace. Assuming the subtitles are accurate, Suleiman told Ibrahim that Hürrem tried to take her own life, as opposed to threatening it. Nigar only knew that Hürrem’s next plan when told that escape was impossible was suicide, so I don’t think that she knew that she’d told Suleiman that she’d kill herself. Valide Sultan, Hatice, and Mahidevran were all under the impression that she’d only had to ask. Even the concubines gossiping thought that Hürrem didn’t have to ask twice to have the Russian concubines sent away.
Things may have still broken down due to her rivalry with Ibrahim (especially in the event of another turning point happening that exposed his brutality, and considering that they were already rivals for Süleyman's attention before Leo even happened I think this is likely) but it wouldn't have been as pronounced as what you see in seasons 2 and 3.
Considering how easily he dismissed Hürrem as a threat when he knew that she wanted revenge for Leo, I wonder if he’d have been even more dismissive of her in this scenario, at least for a time, probably until Suleiman revealed that he intended to marry her, and even then not seeing her as a threat to him but rather as a potential threat to Mustafa if she can influence Suleiman in favour of Mehmed.
Maybe you get a version of Hürrem who gets a wakeup call further down the line as she realises that Süleyman and the dynasty at this point prefer Mustafa over her own sons. And that this is a direct threat to their lives. It's hard to say.
Perhaps a softer, less traumatised Hürrem would be quieter and subtler when it came to gaining influence over Suleiman, recognising the value of being underestimated.
In contrast, an Ibrahim who is more confident about his hold over Suleiman could become even more arrogant, potentially even hastening his downfall. He was always Icarus. It was only ever a question of how long it would take him to fly too close to the sun.
