Hi Mike,
I am a technical trainer with OnSSI.
I can answer some of your general questions, but for specific detailed design info, let's wait to see what our sales engineering team says (John reached out to them already) - they are far better equipped than I to provide answers to design-related questions regarding best practices for storage and archiving.
In your first paragraph, that's one i can knock out - licensing is per camera. The failover server 'becomes' the original (dead) recording server - as far as the system 'sees' it. i.e. no extra camera licensing is required because there are no 'additional' cameras. The cameras just think that the original recording server changed IPs and they send video to the new IP. (if surveillance cameras could think)
I'm not exactly sure what you are asking in your 2nd paragraph, but it is quite probable that your answer lies somewhere between pages 232 and 239 (Failover Chapter) in this document --> Ocularis ES/LS Manual. :)
For your last paragraph, you can record and/or archive to a NAS, but OnSSI does not recommend this practice (see 1st diagram from our training manual below).
For SAN (or other DAS) as primary with archive to NAS, there are apparently no disadvantages! (2nd diagram below) :) ...actually, I'm sure there are some disadvantages. Let's see what my team says... hold tight.
I also forwarded a message to our Tech Support Manager for input (his team sees the results of bad storage/archiving schemes), but he is out today. :(

