I agree with the above comments.
1. When we fly, we always choose Southwest. As noted above, try to print your boarding passes ahead of time so that you can be among the first people to load the plane and will have the option to choose you seats/make sure you can sit together. It is also nice being able to have a seat toward the front of the cabin so you can be among the first to exit the cabin...it seems to take so dang long when you are at the back of the plane (but that may just be because I am antsy to get to Disney

)
2. I have never stayed at the Clarion, but have stayed off site many years ago, and we don't do it anymore. We just have a much nicer time when we stay on property, plus we have the bonus of using Disney transportation which we know is trustworthy. It is just so nice to be immersed in the magic for your whole stay. You also have the added benefit of using Disney's Magical Express (for free!) to and from the airport, and Disney/Southwest will also take care of your luggage for you so that you don't have to worry about towing it around. That said, I'm sure you are going to have a magical vacation onsite or offsite. Since you are going during a busier time of year, I know the Disney resorts are priced up a bit so of course you should do what works best for you.
3. Dining Plan- as noted above, you can only use it when you book a vacation package (tickets & hotel). It is a lot of food, but it is so fun to try all the new restaurants while on vacation, so my family has grown to love it. Our favorites are Boma at the Animal Kingdom Lodge, Coral Reef at Epcot, Ohana (for dinner) at the Polynesian, Whispering Canyon at the Wilderness Lodge...I could go on and on.
4. Tickets- I'm a big fan of the Park Hopper...we use it everytime and have been on numerous trips over the years. It just adds a lot of flexibility into your schedule. Plus some people say that some parks are only half day parks for them...if you decide that is how you feel it is nice to have the option to hop over to somewhere else. I would say since it is your first trip...do the 5 day ticket. There is so much to see and do that you won't be able to cover it all in one vacation! I think it would be nice to have the extra day to go back and see what you missed, especially since the parks will probably be busy..you may want that extra day.
5. Universal- I have done it twice and was not impressed. That said, there are others out there who really like it. If you are into thrill rides, maybe you should go for it. If you look for that extra attention to detail, guest service, and "magic", stick with Disney & would tell you to use your 5th day there...but yes, I probably am biased, lol.
Have a great first trip--you are gonna love it!!! Keep the questions coming, we love helping out here!