A/B testing at the moment is done with a client-side technology called VWO, we are thinking to move on a server-side A/B testing tool but we are still deciding which one to approach.

We load at runtime micro-frontends but not part of the UI inside a micro-frontend, what I’d do in your case is using JavaScript for loading the HTML to replace and then append it inside the DOM.
Another possibility would be shipping both experiments and activate one based on the signal provided by the server.

Regarding dependencies, if 2 or more micro-frontends are using React we are loading for every micro-frontend

I hope I covered everything if not let me know 😄