Hi Tim -
Going back to your original questions ... I can't recall exactly as I have more recently started working on my PHR certification and am confusing the two forms, but I seem to recall that the form stopped letting me add projects once I had satisfied the minimum hours. Whether or not this is the case, keep in mind that there is no "extra credit" for padding this section with extra projects, and in fact I would argue that including extra projects that are perhaps weaker examples of your past work may draw unwanted scrutiny if you do happen to get audited. Even if you can show evidence of 9,000 hours of work, I would suggest selecting the 4500 that are the strongest examples of your qualifications, and don't worry about the rest.
And yes, absolutely coordinate with your references before submitting the application. You may discover through those conversations that one of them doesn't necessarily remember the project or your role the same way you do, and that may have an impact on which projects you decide to include on your application.
Regarding the "lag time" after getting signed up, I submitted my application in early June and had an option of a test date a week later, so it's certainly possible that you'll find something. However, given the number of people trying to get in before the deadline, and the number of people who may be trying to find test dates that let them take advantage of vacation days in December and early January, it's probably a good idea to assume space will be very limited around that time of year in general. My advice would be to try to complete your hours and submit your application as quickly as you can, ideally within the next month or so if practical, as that should give you some additional flexibility (and if there do happen to be slots open in early January, you can just use that time for extra study).
If you're using the PM PrepCast for your hours (I recommend this), you should be able to complete the course within that timeframe provided you are able to find enough time outside of work. The ability to listen as you're driving to/from work or watch a couple of hours late at night definitely makes it easier to work into an aggressive timeline than a traditional course with fixed classroom hours.