I don't think a publisher is necessarily needed for the PU...
As I understand it, CIG are trying to ensure consistency between Squadron 42 and the PU, so that they share the same engine, controls, flight models, etc., which means that a changes made for one product need to be done in a way that will not break or unbalance the other. At the same time, they have focussed efforts primarily on getting the single player Squadron 42 completed, both because it will be easier\quicker to "complete" as well as because it is an easier product to test changes in as there is a smaller number of variables needed to be addressed, with additions for the PU able to be added on\adjusted based upon the "fixed" settings taken from Squadron 42.
What is really needed is for Squadron 42 to reach a point that the shared features can be considered "complete"\"balanced" around the limited functionality required for that game. Once this is done and there is a known "baseline", all the other "proof of concept" areas that have been tested can be added, rather than adding beforehand and then needing to constantly change to align with Squadron 42. Armor, weapons, components, ship handling, etc. can be balanced around the limited set that had been locked in for Squadron 42, and other areas of gameplay that have been teasted (either shown but not included, or included in a very limited manner) would be able to be expanded: The reason that things like Mining and Salvage are so advanced compared to those other areas in the game is likely because there is no link\shared dependencies between the PU and Squadron 42, so development on them won't require many\any changes in the future.
Another reason that Squadron 42 needs to be released is that there are likely a number of "secrets" and features of that game which may be "given away" if the technology\functionality that is shared between them is published before that game is ready: It's also one reason why certain ships which are nominally "complete" (Idris', Javelin, etc.) are not flyable in the PU.
I'd expect that once Squadron reaches a QA stage (Alpha\Beta\?) and is preparing for release, we'll see a rapid change to features in the PU as "non spoiler" elements are expanded to cover the additional ships\etc. that the PU has, with the "spoiler" related ones hitting at\soon after Squadron 42 releases... With news that Squadron 42 has been removed from the store for "price adjustment", I wouldn't be surprised if an official release date for it will be made public in the next few months.