I suspect a large majority of the issues you brought up with splitting can be addressed by legislation.

Off the top of my head, something along the lines of having a mandatory period of time where the company being acquired must be kept separate enough that the merger can be dissolved in a timely manner should it be deemed necessary. How long that window should be would be up for debate but personally I'd argue the window should scale somehow based on things like the valuations of each company involved, headcounts, and maybe even competitive landscape.

An example would be requiring both companies to perform a pre-merger assessment to determine positions/roles, technology use and regulatory requirements that must be maintained for a clean separation and creating a plan that gets filed with the relevant agencies to be used if/when the merger needs dissolving. If the time comes that they need to dissolve the merger but they don't maintain that separation, they still have to do it and you slap a fine on them equal to some percentage of that mergers cost to be paid by the parent company.

Hell, that idea alone would accomplish both making it harder to merge and easier to dissolve the merger.