I believe this is a viable path forward. If Hey were to charge an in-app-purchase of basically any amount they would be okay. As long as they aren't telling users to go to hey.com and pay there instead, they'd be able to. Google does this for YouTube Premium right now, for example.

The catch and the thing that Basecamp probably wants to avoid is that they would have to have a higher sticker price in iOS, and have to build support for it. The current price on their website is $99/yr, which would be an interesting iAP price too (so they'd probably want to go full monthly).

No, but it does prohibit you from telling the user that they are paying 30% extra, or telling them they can get a discount elsewhere.