These are great questions. Let me see if I can answer them all.
You cited the APs and controller count, but will you also be adding in switches? The scaling numbers are testing with 20% of the device count as switches (this was determined to be a median value for the number of switches). The tests also account for continuous client traps and syslog messages per second (Table 3 of the 7.6 server sizing guide). If you do include switches, they will also be counted in the device count.
Your concern for lag is valid, but keep in mind that as long as the APs are behaving as thin APs instead of autonomous APs, AirWave will primarily contact the WLCs and switches for data, so as long as the controllers are not overloaded and responsive, there shouldn't be much lag except for time of SNMP poll to time of SNMP response from controller. You also have the option to override the polling cycles to stagger the data collection depending on your data priorities. And SNMP traps will add additional data points to provide more realtime client tracking.
When it comes to licensing: for 2600 devices, you'd purchase a 2500 device license + a 100 device license, and then consolidate the 2 licenses on the licensing site (which should be fairly easy, but I've never tried).
I don't currently have direct data to describe the traffic over 3000 APs through 20 WLCs for 6000 clients, but a customer in the education space with a deluxe-sized single server AirWave has roughly 8000 devices, 10000 clients pushes an average of about 10-15 Mbps.
In regards to HA, we have a Failover solution currently, but it's a passive solution. This will be changed in the future as there's research being done on new architecture that will improve the HA design to make it an active solution (instead of reactive). The user guide should have some information pertaining to the Failover. If there's anything unclear, you should be able to reach me over email (I'll send my email address in a private message).
AirWave will directly communicate with the controllers. There's some light ping tests to all devices, but primary traffic will be SNMP, and then scheduled firmware upgrades are done over FTP/TFTP.
Major releases are twice a year. Each major release is followed by patched releases that are typically in ranges of 2-6 weeks apart. Patched releases are typically done based on customer needs. To give you a better idea, 7.6 was released 3 months ago, with the current release AirWave 7.6.3 having followed 7.6.2 after 6 weeks.
Several existing customers do a central deployment (this works as long as the communication between locations doesn't incur too much additional latency). Central deployments are advised if you're the solo super-IT. If you've got a team, you may go the Master Console route where you have a Master Console watching smaller AirWave servers to yield a larger network view, while site admins take care of individual distrubuted AirWave servers.
Let me know if you have any additional questions.