We do indeed have a rough ETA on content updates! Please bear in mind that the schedule is by no means definite at this stage, and we are a tiny team, so we could encounter delays. At the moment, though, we're on track to release an our first version of Free Mode next week, and at least part of Chapter 4 the week after.
The in-game GUI is currently placeholder and subject to change. That being said, we do have 3 heroes confirmed at present, and others in the early testing stages. Whether or not those make it into the initial release remains to be seen.
In terms of the number of chapters, we can't give a final figure, or confirm what all the chapter maps will be used for (some may be for side stories or challenges), however we can confirm that the main story will include a minimum of 10 chapters.