1. Khmer might be anywhere from Ancient to Medieval
I'm not sure about that, unless by 'Khmer' they put Funan or some archeological cultures, Khmer empire itself is fully in (European) medieval era.
(Mekhong civilization cradles? what races shall they pick?
I don't they will pick aby. It seems ancient era 10 slots are Olmec, Nubia, Egypt, Mycene, Assyria, Babylon, Hittites, Harappa, Zhou and Phoenicia. With 10 slots per age only the most famous civs get in.
3. What makes Myceneans different to anyone else? Particularly Greeks?
They are very well documented, advanced, separate civilization. They are separated from Classical Greece by like 600 years, had significantly different language (though still classified as Greek), completely different writing system, and a lot of differences. Really the only choice for recognizable bronze age European civ
is them or Minoa.
4. Where's Minoans? Should their UU be 'Minotaurs'?
If Mycene get here, I'm pretty sure Minoans don't.
5. Ming is Renaissance China.
Not necessarily - Ming started in 1368
so they'd qualify as medieval civ, of course if we want to use European terminology. Also, in this 'medieval' first half Ming was flourishing, while in 16th century it started to rot. On another hand devs could use Zheng He's voyages as basis for renaissance era vision of colonial China, which would be amazing.
Really it's up to devs if they choose Ming to be Med or Ren.
6. Levelling up as Vikings/Norsemen might permit players to sail across a big ocean and found a new city on a different 'new world' early on, and super shock troops named Berserkers with strong attacks but bad defenses.
Honestly, at this point I'd love to see any Viking unit other than overused berserker. Also, that trick with different continent city never ever worked in civ (it was always trash ability) so...
7. Where's Phoenicians? they're a big seafarers and it is said they came to America before Norsemen do.
Phoenicians were mentioned by devs IIRC, I'm pretty sure they will be ancient era, if we measure classical era by 500BC then they fall entirely out of it.
Anyway, half of the world claims discovering America before Columbus - no proof at all besides Vikings.
8. Did they confirm that Siamese and Burmese will or will not appear ?
No. I think if they appear then in renaissance.