If the invite comes later than January, something has to give. A company failing to deliver on a promise and then trying their best to make up for it is nothing new. Most classy move would be a refund on the sub price without freezing the sub. 5 days left without any communication as to why this taking so long - I expect something good if it comes to that.
I think it's pretty certain the engine launch will cause a huge surge of players, probably mostly returning ones; the question is whether they stay for very long. I know a lot of people who quit because they basically played the game out and got bored, but would return now and be able to play the game fresh, and they're looking for an excuse, which the engine provides. I know from reading comments on the videos/screencaps that some people expected a full upgrade of all art and such, and those people might not stick around; some people who are obsessed with the balance problems and such might not stick around; but FC will be infused with cash from the initial wave of subs and that might allow them to do a booster or something. Let's hope.