How to tag

1. Use the key 'level=x' (see http://wiki.openstreetmap.org/wiki/Level) The level=0 represents the ground floor (for historical reasons, we use British English in tags). Then, in your case, I would create 3 nodes, one per address, attached on the building way (my preference is to put them on the front side where main entrance(s) is(are)). Then add the tags for individual addresses and the corresponding floor with 'level'.

2. see answers on this identical question

3. again, create two elements e.g. two nodes inside the building area where you add the appropriate 'amenity', 'shop' or 'whatever' keys plus the 'level' for each of them.

4. if by "urls", you mean permalink when downloading a zone for edit, open the 'download' dialog then select the tab 'Bounding Box' and paste the url there.

Personally when a building covers multiple address, unless they fit very well on the polygon, I always use a range, but then I live in a city (Montreal, Canada) where having 3 to 5 4-digit housenumbers on a small building is a common occurrence!