Hi,
1: Find GameObject action is a bad action because Unity internal api to find a GameObject by Name is known to be very slow, so no, this is not a good idea
use ArrayMaker or FsmArray to store your tiles, use a Hashtable with ArrayMaker or two FsmArray, one for the keys "string", and one for Values: the GameObjects representing the tile.
but all in one, Tile Based Games requires some heavy core features, maybe you'd be better off using a ready made system and build the custom actions to control that system within PlayMaker. It depends your level of expertise really.
2: Can you describe what you mean by preview? maybe an example in a real game or something?
3: MVC works well-ish, because of the component based Unity approach, it's not really a clear fit and so you'll likely end up with a View/Component with the Model being spread out everywhere, but if you are serious about it, it can be 90% in the Component. For an exampl at what MVC would look like ( At least my version of it), check out the "ugui IOS View Controller". If you are familiar with xCode and IOS development, you'll find the same patterns of communication and MVC setup basically.
Bye,
Jean