1) When will it be released? They have only stated Holiday/Winter 2016. This is from Late October to End of December. There has been no solid date given as of yet.
2) Eshop or Physical? All main titles get physical releases, seeing as this is going to be a main title, I don't see why it wouldn't get an physical release and either at the same time or shortly after, get a digital release.
3) Diance, Hoopa, Volcannion: Like every other Mythic/Event Legend, pretty much no. They are event exclusive and by the time Sun/Moon releases all three of these will have been released on XY/ORAS. Maybe much later down the line, like when Gen 6 needs a remake, we may see them get the Deoxys treatment and added in as a post game story where you get a chance to catch them. Other wise, I don't believe we'll see them again until GF releases an event to download them.
4) Squishy: Squishy is Zygard, or a part of Zygard to be exact. It is not a new Pokemon, in fact sources have said that it is not a Pokemon at all. Zygard's Core (Squishy) and Zygard's Cells aren't considered Pokemon, but when they combine, they create Zygard 10% (the doberman), Zygard 50% (the snake we know), and Zygard 100% (A gundam looking humanoid like Zygard). As far as we know, Zygard's Core, Cells, 10% Forme, and 100% Forme as well as Ash's Greninja are Anime exclusive. Not much reason to assume that they'll add them into Sun/Moon since the two major aspects for it aren't Pokemon but are required to be there for the form change to work. Their Form change also isn't very game friendly without needing to recton in a new control device, similar to Kyurem's control device or Deoxys' meteor. Bottom line concerning this topic however is personal opinions. We have no information on whether they could or could not appear. There is mild conflicting information.
Beyond Sun and Moon's name, we know nothing. They literally only gave us the name and a vague release. More info is going to appear as it gets closer and everything other than these two facts are pretty much all fan speculation. One thing we've deduced is a new Bird Pokemon that doesn't match any existing birds thus far. Some are jumping into the "based on Hawaii" bandwagon due to some very convincing evidence from concept art shown. Neither of these two have been officially cleared however. GF could always come and say that the bird was a beta design that didn't make it to end product and give us a reason for coloring the vehicles while basing the region off of a completely different place than speculated.
tl;dr we know basically nothing