It is probably best to think of Metro Transit's fares as purchasing a period of time in which someone may ride Metro Transit's system, within the fare category. A regular ticket will purchase 2.5 hours of transit on Metro Transit; if one would go on an express bus from METRO they would have to pay the additional fare. One cannot step up from METRO to Northstar with a paper ticket; however, a Go To card will automatically "step up" the fare to a Northstar fare, only charging the difference ($3.75) between the light rail ticket and the Northstar ticket. The one exception to this rule is if one transfers from Northstar to Northstar; at that point a new full fare is charged and the 2.5 hour timer is reset.
One way and roundtrip Northstar fares can be bought at any light rail station. And while it's not advertised, a round-trip Northstar fare is listed on the machine (when purchasing it) as valid for 24 hours from the time of purchase. In my experience, it expires at midnight on the day of purchase.
Northstar tickets are valid for light rail and bus trips within the validity period. If one wishes to play it "safe", one could purchase a light rail ticket at St. Paul Union Depot and then buy their round-trip Northstar ticket at Mall of America. That round-trip would be valid through the end of the day, including transfers to/from light rail and buses. However, I would not anticipate any issues if someone were to purchase their round-trip Northstar pass at Union Depot and shoehorn it into a day pass instrument for light rail as well, as all of our trips would be within the validity period of a round-trip Northstar pass.
There would be absolutely no need for a downtown zone fare while in Minneapolis. Even a one-way ticket bought at Big Lake would cover the trip to wherever we wish to eat that evening.