I guess I'm not following your math.
MNSSHP tickets run about $60 a person, so you are going to spend more that $100 for that. I don't know exactly what date you are thinking of attending, but we did not find short lines for attractions last year on October 29. In fact, the lines were longer during the party than they were during normal operating hours on a non-party day. This party has gotten too popular, and I think Disney sells as many tickets as they can, rather than limiting the ticket sales as they did in the past. So, you could find yourself in a big crowd.
Also, if you do go to the Fort for looping, that is going to cost you either in terms of bike rentals or golf cart rental. So, that would have to be factored in to the cost of the carriage ride. For two people, I am not sure that this would end up being much cheaper than going to MNSSHP.
I think you're right though, the value per hour is better with MNSSHP.
I think if you visit the Fort, you should do the Haunted Carriage Ride, and watch the EWP and Fireworks from the beach. Also have some drinks at Crockett's Tavern- but remember, they roll up the sidewalks at the Fort at 10 pm.
TCD