Yes you can preview the house just like you can preview all the houses (even ones not in the cash shop). Go to Morthal then head out towards that miniboss on the river (mother of sorrow or something like that), a little past there is a bridge that leads up to the house.

As for not understanding the scrying game, google Alcast, they have a nice guide about it.

As long as the event bar reaches 100% everyone will get all three rewards, even if they didn't participate in the event.

The bare minimum requirement is having purchased the Greymoor chapter.
As long as you have the chapter unlocked on your account, AND the progress meter hits 100% before the event ends, then you will "earn" the house along with the Greymoor community.

It's a community-effort event, reliant upon the participation of the community as a single collective unit.
Individual contribution from within the larger Community is not assessed, so not everyone needs to participate; it is the overall progress of the Community as a whole that moves the bar, although individual participation obviously adds to the progress of the Community. The fewer who sit-out, not participating, the better.
But not participating will not bar you from receiving the reward if the rest of the Community earns it, as long as Greymoor is unlocked on your account.

As long as you have done the initial Antiquarian quest, which grants access to the scrying and excavation skiils, you can add your effort to that of the group's. Excavating antiquities moves the progress bar. Quality is irrelevant.
So one can simply dig up green items to contribute.