Re: 500-year half-life? Yep.
Were this DNA archival process ever to be used, there is no reason why the archive couldn't be based somewhere cold - much like the Svalbard Global Seed Vault. (https://en.wikipedia.org/wiki/Svalbard_Global_Seed_Vault#Construction )
Certainly. And your average optical or magnetic medium might last quite long under the conditions of reduced ambient temperature and in oxygen-free, low-humidity air. With the added advantage of not being literally digested by any fungus or bacterium which manages to sneak in. Of course, these are rarely the conditions in which the real-life "archival" medium is stored - so YMMV.
Like most research papers in flashy journals, this is a nice idea, demonstrated through hard work of many very smart people using cutting-edge technologies, and then served as a 5% solution in pure hype.
I'll be very happy if it eventually leads to a practical storage product - but this is rather unlikely for a variety of reasons.