I just got caught up on your PJ (thanks to the lovely Orlando monsoon going on at the moment that has me trapped indoors at work...hope no disbrides had an outdoor wedding planned today or yesterday!). I think you should go with the castle wedding. Your 2nd option just seems like way too much and even if the castle is under construction, I don't think they would put so much of it under construction at once that it would look really bad in pictures and make all of the park guests angry. When they do construction they usually limit the areas they work on at one time.
As far as theme park guests, if the ceremony is at 7 and the park opens at 9 they might hold the guests with early breakfast reservations until 8. That way they'd have minimal contact with you. If your ceremony is at 8 you'll probably have more of a problem. I can't see them keeping all guests away from the castle area if they have a breakfast reservation at that time, although they would probably escort them up the other side of the castle or direct them to the path next to Sleepy Hollow/Tinkerbell's Treasures so they'd only see your wedding from a distance. You would have to ask Disney specifically about that and ask them to state very specifically which path the park guests with reservations will be taking if it is going to bother you that they might see your wedding going on. You might want to walk it yourself to see if it is acceptable. I'd also get it in writing just to be safe.
If you hire characters for a special event Disney is good about keeping them just with the group paying the $ for them so that probably wouldn't be a problem and I doubt they'd let guests be on Main St. as you were coming down it although I wouldn't be surprised if you would see some of the cast members that are opening the park that day curiously peeking at you out of shops, etc.
What about doing your welcome dinner at LSS instead of Napa room and having all of the guests stay at Beach Club? Then you could have the scuba Mickey and do a dessert party too. This would work if you did the cruise option because you could do LSS/DP/Beach Club character breakfast/cruise. It would be less convenient with the castle option but you could get a bus to transport all of your guests over to MK and do LSS/DP/castle ceremony/reception at CG or Napa room or wherever else you'd like. Either way it probably wouldn't cost too much more since you would be cutting out Napa room. You might need to think a little bit outside of the box too. Didn't the couple that actually got married in front of the castle do the reception before the ceremony? I know some couples have. You could cut out the welcome party and do a LSS reception/DP/cruise ceremony afterwards.
I love the Belle dress and it makes you look like a true princess. I can see why you would want to wear it. It would be perfect for the castle wedding but if you don't choose the castle why not just wear it to do a castle photo shoot followed by a breakfast/brunch reception? If I had that dress I would wear it regardless of the location I was getting married at but I haven't stumbled upon any great bargains like you and don't think the quality of the Disney dresses is worth their pricetag.
I agree with everyone about the cover-up for the beach ceremony too. With all the money you are spending on your wedding I think you should go with something a little nicer.
I think you need to sit down with your DF and really decide what is the most important to both of you. You can always save the other elements to do with a vow renewal.