Ok, so in today's build, all dungeons except Assault on Violet Hold are open on Mythic difficulty. When you create a keystone, either using the purchased item or by beating a higher-level Mythic dungeon within the timer, it should randomly point to any of them.
There's a bug with the placement of the Challenge Pedestal in Court of Stars (it's, uh, not subtle) which we'll try to fix shortly.
Tuning may be a bit scattershot on this - I'm definitely interested in hearing which dungeons seem easiest, which seem hardest, which have the most or least strict timers, etc.
Pretty comfortably beat the timer (6-8 minutes remaining I forget, logs are weird) on a GR6 Raging/Necrotic Neltharion's Lair. Challenge at i845 is probably too high for the i840 loot at the end but the instance at GR6 felt pretty well tuned for that ilevel (would be a different story with Tyrannical of course).
The loot levels you're seeing aren't final - you're currently seeing the same base ilvl regardless of completion level. I'd expect more like 855+ from the chest at the end of a level 6.
In general, I'd say you're doing these quite undergeared if you're pushing levels 6 and 7. The dirty secret of these tests is that we're using great players as guinea pigs to get a sense of how far beyond a given numerical threshold skill can take you. Some affixes (e.g. Volcanic) are more skill checks that you can play around. Others (Tyrannical most notably) have less finesse and don't let you compensate for lacking gear with skill in the same way.
In terms of tuning, for the gear level (845) to which we're scaling you, I'd say "par" would probably be around a level 2, in terms of a point where it feels about like what a base Mythic dungeon would feel when you're doing it in Heroic dungeon gear.
new affixes are bolster & overflowing currently, as a random aside the trash contribution requirements to most of the dungeons we have completed thus far seem really low which is going to result in a lot of skipping even w/o using invis pots or a full stealth group to get by trash
Would love to hear more examples. Squishei's linked videos above seemed pretty close to the mark, with the bar filling up during the bit of trash before the final boss in each dungeon. Eye of Azshara is a really tricky one because of how wide-open the layout is, and how much you can avoid without having to use stealth/invis shenanigans. In general, the Enemy Forces step is tuned around killing the vast majority of the unavoidable enemies on your way to the boss. If you can slip past patrols naturally, then you're rewarded for that (but of course if you wipe you have to deal with doing so a second time). Stealth and invis may let you selectively skip a couple of packs to earn your contribution requirement on easier ones, but that feels like an acceptable level of benefit from utility as opposed to a gamebreaking one.