So excited for this, really excited to dig in and start kicking the tires. One question, since it looks like steeleduncan is answering questions in here: How does the realtime typesetting deal with large documents? I can't tell if it can keep up because it's only re-calculating a segment of the total document, but I've been working with LaTeX mostly for book-length documents; my chronic annoyance is the minute-plus compile times unless I break it up and only compile a chapter or two at a time. A realtime option would be a huge benefit for the final typesetting runs where I'm trying to fix image placement and orphans and whatnot, but I don't know if that's asking too much of the engine.
When you move to a new part of the document you will see "Caching" briefly whilst Texpad prepares itself, then as you say, it will be incremental typesets from then on.
What you are suggesting should work well, if you have troubles with that in the beta please get in touch with us at support at vallettaventures dot com with the document and we'll take a look.