> The legal system, including the discovery process, and basic accounting to calculate when the selling price is lower than BoM cost.

So price controls is your answer? The Costco rotisserie chicken is illegal monopolization in your world.

> What mechanism can the regulators use to prevent competition going out of business while maintaining the artificially low prices?

The legal system, including the equitable power to split up companies under antitrust law.

There is a reason that the proactive approach has been roundly rejected by courts in the modern day, despite Lina Khan’s best efforts to push it. Despite your comment earlier about preserving competition being “easy” (preserving competition via regulation is never easy) and preserving low costs being “hard” and on “shakier legal footing,” the test for antitrust remains consumer harm, not competition.

> The legal system, including the equitable power to split up companies under antitrust law.

And how well has this been working out? There hasn't been a real threat of forced divesture since Microsoft's anti-trust case back in the 1990s, and large companies are willing to pay fines that are dwarfed by the profits earned from destroyed competition. How does paying a fine undo consumer harm?