Is it possible for a merger to include a company that has not yet been acquired ("started")?
The rules always refer to mergers as involving anyone with open slots or the owners of the two companies, and the acquistion rules refer to starting new companies. This leads me to believe that unacquired companies do not actually exist yet, but are only potential companies, and therefore can't be included in a merger. (If this is allowed, it would essentially open a new path for those invested in Merger R&D rather than Slots to pay to acquire a company.)
While the English rules don't explicitly say that mergers have to be between owned companies, they certainly strongly imply it in a number of ways (including the fact that no mention is made of how to handle the case where one or both of the companies are unowned). So, I have to assume that is what was meant. Certainly that's how we have been playing it.