Start doing your research now, which it sounds like you're beginning to do!
No, there is no dining plan. I would suggest venturing out for dining at least some! You can find restaurant prices for Aulani & the Marriott & Four Seasons if you do some searching. Despite that I adore the Marriott SO MUCH, I do not dine there nor Aulani. We prefer local places, and the beach side place at the Marriott (Longboard's) has been awful each time we've dared to try it.
I stay (owner) at the Marriott next door (Marriott's Ko Olina Beach Club), and also lived on Oahu in the mid 1990's. We come from the East Coast, and stay for 2 weeks typically every other year (but next year we're locking off our 2br condo, so first 2 weeks it's just me and DD in the studio, then the following 2 weeks my DH and DS will join us).
I highly recommend a car the whole time. We use ours almost daily! If you're renting points, you will be treated as an owner, so you shouldn't have any parking fee.
I like the summer; that's when we always go. Spring break is tough, since coming home we have to adjust to a 6 hour time difference. At least in the summer, my kids have time to adjust back!
Aulani and the Marriott are ALWAYS pretty full. So that won't make a huge difference in your choice of time of year. We never experienced a hurricane, both living there and visiting.
Right nearby in Kapolei, you can shop at Costco (they sell liquor too, plus lots of Hawaiian specific goodies), Super Target, Super
Walmart, and Safeway. We do our main shopping at Costco, then fill in at Target (their prices for most everything are comparable to here at home). Safeway is a little pricier, and I don't shop there much. I plan a few meals, and bring spices and condiments from home.
We tend to go out and about during the day, but prefer to grill dinners at night at "home". That being said, the Marriott has far more grills than Aulani does, and they're available at any time. I believe Aulani only allows guests to use them in the late afternoon/evening. Aulani is much more compact, so finding pool chairs can be a bit of a challenge.
Start researching flights at 10 months out at least, then when you find something good, grab it. We use miles, and typically have our flights booked about 7 months out.