This would feel contrived if the OP stuff was illogical, the complementarity of men and women wasn't such a big theme in WOT, if Siuan's relationship to power wasn't a big aspect of her character development and so on etc.

As it happened, it's a well constructed aspect of the plot. Siuan came from nowhere, powerless and was thrown away from her own home by law. Thanks to her ability to channel and her strength in saidar foremostly, she climbed fast to near absolute power at a young age, then because she thought herself all powerful and failed to watch for traps well enough, she lost it all. The character is set up as an object lesson to Egwene, who got a similar but even more dangerous rise to power. Healing Siuan justified having her as Egwene's advisor. Stilling served to show Siuan herself that even without the OP, she's still the same women, with the same strong personality and undomitable will. It showed her her real strengths, and that her real weaknesses were not primarly having lost the OP...

As for the difference in Healing a man or a woman, it follows the logic of the OP: each half completes the other, and each is an impassable barrier to the other half. Nynaeve's repair with saidar worked perfectly as a "container" for saidin (as it did for Rand at the Cleansing), but it doesn't work perfectly for saidar, and a significant amount of saidar energy passes through it and remains inaccessible to Siuan. As RJ confirmed, Flinn would face the same issue healing a man. RJ even showed more à propos in where he introduced this in the plot, as it happens in the book where Rand starts stagnating in his knowledge (after great progress under Asmodean) and being more and more incapable of finding female channellers he can trust.

As for Nynaeve, it also serves as a lesson to her, beside introducing a major progress in her skills. She's prejudiced against male healers (mundane ones), and her massive success doubling as a failure with Siuan served to make understand down the line that she'll never reach her full talent as a Healer unless she starts collaborating with a male healer.

Another aspect of the plot device was to introduce the notion of experimenting with the OP, being creative. What Nynaeve did triggered a renaissance of the sisters' desire to experiment. Nynaeve has half-healed the Tower of their apathy and fears toward saidar research.

So yes, a central aspect of it was introduced to justify toppling Siuan and creating the circumstances to raise Egwene to the Seat. But RJ did his homeworks, and made it all happen through story devices that fit for the development of all character involved, with the worldbuilding elements, and with his greater and smaller themes. That's hardly "a contrivance", or else any plot development is...