I think any ethics concerns are absurd if Mercy is meeting the letter of the law. The whole insurance and lab system in this country is already ethically bankrupt. As long as Mercy is keeping it legal and is good for my patients, that's fine.
As far as Mercy dumping low volume providers, you have little leverage as a single small DPC but if a DPC network contracted with Mercy as a group, then you have lots of leverage. We'd need to pay a lawyer to check for antitrust issues and we'd likely need a more formal business entity than a Facebook group to negotiate and sign deals with labs.
Running our own inhouse labs will be a good option soon but none of us are big enough yet to make that economically viable. There are several of us within driving distance of each other that could pool our labs but then you get into whether or not you're running a reference lab with much more legal, regulatory and liability issues to sort out. I loved having an inhouse lab at my old insurance practice.