Heather might be a wild one, but at least on the share route (unless it was rewritten) Heather definitely comes across as really liking the MMC (unlike the two other routes), you should know best of all.

On share, Heather should get quite involved in the "games", giving ideas and partaking. As I mentioned in earlier posts, Heather and Debbie in special should be at the forefront of the girls the MMC impregnates on the share subpaths. There should naturally also a subpath were the MMC impregnates his wife, only her and another were the MMC also impregnates the other girl(s). But that is sharing route.
On cheat and cuck, Heather is much more a "force of chaotic disruption"
I agree, this scenario is much more aligned with the cuck route. For sharing to work, IMO Heather and the MMC should be "having fun"

themselves, either in the same room or with videofeed. Maybe Heather riding the MMC and once he admits to like watching, Heather tells her bestie to really have fun and she would take care of her hubby. And that the FMC should look if she likes it too. Then the MMC and Heather get really wild themselves, maybe telling each other they wetted their appetite watching the FMC and now full speed ahead.

Something like this could work for sharing.