You can't sell the platform if it's heavily integrated. Whoever buys it will just have to rewrite it from scratch then.
Do you think lawmakers care? There is a fair bit of public goodwill for acting against big evil intl tax evading tech companies...so I think they actually could force this.
Not necessarily. My understanding is WhatsApp basically uses the Signal protocol right now, Signal itself is open source, so I assume an acquirer could just stand up some new Signal infrastructure and get 80%+ of WhatsApp functionality without much redevelopment.
WhatsApp is a phone app. Even if Facebook heavily integrates the messaging infrastructures, the problem an acquirer has is porting the existing users over to a new messaging infrastructure. Signal-based infrastructure is (relatively) turn key, most of the software is already developed, deployed, and tested. After you have that, the main thing you have to do is push a new version of the app out to all the different app stores that uses your new infrastructure. Bam, you're done.
I am simplifying certain things (there'd definitely be a somewhat complex transition period where your new app would have to support both infrastructures), but my main point is that this integration is not as big of a barrier to re-separation as it may seem.
If you're selling it, it would be your job to port it. This wouldn't be the purchaser's job, it would be Facebook's job. It would be like expecting someone to dismantle a bed your selling on eBay. No one in their right mind would agree to dismantle it for you unless you were giving it away.
All the other protocol bits are very much WhatsApp-specific.