Aesculus glabra - that's plenty of questions! I do have answers to some of your questions but the rest will have to wait.
Pollux will not replace Primus. Pollux is a completely new and free bonus engine you get, not from lotteries or by purchasing, but by watching one video. After this, the next videos will upgrade your Pollux engine throughout the round.
One big decision behind Cargo is - Pollux can then be used in all Scenarios. Otherwise, it would not benefit American Dream players. I'm not familiar with the development side if it would be someday possible to go with an optional passenger bonus engine, but as players, you always have the right to make a wish!
Base numbers and max stats - the team should be able to deliver more info in the near future. Some things may be best to leave as a surprise - the same goes for how you can upgrade the engine. Prepare to be surprised! It's quite innovative and fun!
Pollux will be your trusted companion throughout the whole game round, so coupling will not become an issue.
On videos (amount of videos), players have been pushing for an increase based on the comments here, and our team is gathering similar feedback from different domains. It's impressive and the feedback has been heard. I do not have more news at this moment.