NW's policy isn't much different than you'll find anywhere else. You bought the non-refundable ticket, and if you did it online, clicked the box indicating you read and/or understood the fare rules. They're always glad to collect the $100 change fee and charge you the difference between the fare you paid and whatever the current fare in F is, so technically you can pay for the upgrade.

If you'll be Silver by then, consider burning 15k miles for the upgrade. Quite worth it at Silver for 2 flights of decent length. SAN-DTW will be a tough upgrade for Silver, the late flight to MCO, as you suspect, will likely be much easier, but no guarantee.

What did you expect NW to do? The F fare is likely $1000 or more and I would guess your ticket is cheap. If you could simply buy a ticket for $200-300 and then call and pay a $100 upgrade fee, nobody would ever pay for F. That would make no business sense.