sdwdate does not use average. It uses median. Average would be really bad if a single remote clock was off by a lot, it would dilute the result. The average of -600, -1, -1 would neglect the time sources that were right and neutralize the sdwdate trust distribution feature.
I don't know. I wonder if we can get "looks as accurate as NTP (or anything other popular nowadys) so we can blend in the group of host Tor Browser Bundle users.
Nitpick: Tor. Whonix only by extension Whonix. Just stressing this to not create the impression of limitations introduced by Whonix.
I don't have the reference handy, but the 30 minutes tolerance might be fixed in recent Tor versions.