That chart is wrong, or mis-labeled. The left column looks like what domestic segments can be upgraded with miles, the right column what international segments can be upgraded with miles. (That chart is also not authoritative - the authoritative answer can be found at
http://www.nwa.com/worldperks/elite/...pDomesUpgrades - believe me, I've gotten plenty of upgrades on L/T/K fares.)
Quoting from that page:
The following fares do not qualify for upgrades: charter, bulk, VUSA passes, VUSA fares, travel-industry free and reduced-rate or standby tickets, student, infant or child fares, companion offers and group fares. Some ticket designator programs – including N/GP/X/C – may allow upgrades; check with Northwest Airlines for details. Upgrades are not permitted with select promotional offers and other fares that may be specified by Northwest Airlines. Only flights that are eligible for WorldPerks mileage credit qualify for Elite Qualifying Miles.
Yes, with Gold vs Silver you will get a higher mileage bonus, and higher upgrade priority. Will make a difference on short flights, but frankly the way DL has been handing out MQM's there's so many platinums that you are still going to have basically no chance on any of the heavy routes (transcons, etc).