1. There is only one week between the abstract and paper deadline, how will I have time to write my paper in between?

For ISMAR, the abstract deadline is just informative and not used to review your paper. It does mean that you won't get any feedback between both deadlines, you should just proceed to write your paper for the paper and poster submission deadline, and just enter some basic information on the PCS for the abstract deadline.

2. What the difference between AMH and S&T track? I wrote an application paper in architecture, I'm not sure where to submit it...

In essence, the S&T track is directed more towards engineering, system, algorithms or empirical studies, and applications in a broad range of domains (e.g. Medicine, Engineering disciplines, Military, Architecture, etc). The AMH track is more focused on conceptual aspect, design, theories & philosophies, experiential aspect and social implication for a range of specific domains (like Art, Social science, Architecture, Product Design, Entertainment).
If you have a paper about architecture, if the content is more about the AR system or some novel AR rendering techniques in architecture, the S&T will be more appropriate. If your paper is more directed the conceptual aspect of using AR in architecture or user-centered design, longitudinal evaluation with it, AMH track will be a preferable choice.
In any cases, the Program Chair will assess early on after the deadline submission is the paper correspond to the adapted track, and will notify you to potentially transfer to the other track.