1) If you have a costco membership, you can book through them and get quite a bit more back in the form of a costco gift card than other TAs will give you in onboard credit (turns out to be about 9% of the cruise cost minus the taxes and port fees, almost double what TAs give). If you aren't, it might be worth it to join. We live two hours from one and found that our membership has saved us so much in rental car discounts and credit from the cruises that it was worth it to pay $55 a year to join. If you don't like this option, then find one that gives the onboard credit. It also gives you someone directly to ask questions.
2) We love the late dining. Last year my kids were 8 and 9, and it worked perfectly for them. It gave us time when we came back on the ship for them to go to the kid's club (which they loved!) or swim or ride the AquaDuck for a bit before dinner (they were both almost empty during early dining time). We didn't feel rushed to get back on the ship and hurry to get ready for dinner. We never missed anything by doing this. If they got hungry before dinner, we would grab some chicken strips or pizza to split up on deck or room service for a snack. Also at the later dining, the kid's club will come gather kids who want to go to the clubs, so they don't have as long of a dinner to sit through. Our servers brought the kid's plates right away and by the time they were done, the counselors were there to take the kids. They loved not having to sit that long, and we loved having "family dinner" then "quiet dinner" all in one, lol. I would assume the dining room is calmer and quieter, but I have never done first seating either for the comparison.
3) The hardest thing for me to get over when comparing it to WDW vacation was the slower pace of everything. There is no rush rush rush to be places. At WDW, you need to plan an hour and a half to get to dinner.....on the ships, it's a 5 minute walk pretty well anywhere. We quickly learned if we came to dinner about 5 minutes after the official time, there was no line, and we were quickly seating at our table. Know that the midship elevators are the busiest and often it is worth the time to walk down to the forward or aft elevators when there is a crowd. Also, room service is AMAZING! I love that I can call and have food delivered (in my defense, I have spent all but the past two years of my life living where NO food was delivered so that may be my fascination with it). If you think (even a hint), you will want to cruise again in 2 years, book a cruise while onboard. The benefits are great (10% discount, $100 on board credit if 6 days or under, $200 credit if over 7 days along with only a 10% deposit for 7 days or more), and well worth it. If you decide later not to go, as long as it wasn't concierge that you booked, the deposit is refundable.
4) Our first cruise was an inside cabin and the last two with verandas (the next two in Oct are also verandas). Given that you have a cruise that has 3 sea days, I would vote for the veranda if you can afford it. If not, an inside room isn't that bad either (esp on the fantasy with the magic porthole rooms). That would be a personal (and budget) choice. Our kids did enjoy sitting out there as much as we did though and having some quiet time.
No matter what you decide to do, you will have a great time. There is so much to stress over and over plan, but after the first one, you will realize it is so relaxing.