At Marazul or AllWest you will need a car - they're approx. 45mins. from town. Everyone covered the dive operations in the area pretty well except Marazul has an arrangement with a local shop for tanks etc. for their guests to use also - there or elsewhere. It might possibly be the Go West shop at Lodge KH also.
One thing about diving at Marazul - since it's up on a cliff - this is the way down.
https://imagesus-ssl.homeaway.com/mda01/b00ca1dc-f145-472f-aa4d-6ad33427e04b.1.10 better than it used to be - IIRC they added the 8 steps almost into the water recently. I've dove near there but not at the site - someone else posted it's not the best in the area - kind of between two better sites.
There's a lot of steps at All West also down to the beach/dive site. And the shop is on the same level as the apts. on the hill. There are other locations on Curacao that are flatter if that's an issue for you. As I think about it, much of west Curacao is elevated above the water - we stayed a few days in Lagun 60' above it - ladder down to get in the bay but we looked at carrying tanks back up and thought - no way. The beach entry/dive site is two blocks south at Playa Lagun. Good dive, there's turtles hanging around as you exit the small cove - fishermen clean their catch there. About 15mins. from Marazul - you'll also pass Playa Jeremi - one of few Curacao dives with no on-site facilities and IIRC Cas Abao is in the same general area - maybe a little farther south past Lagun.
Boat dives from Go West at Lodge Kura Hulanda go to some of the better dives on Curacao - Mushroom Forest, Watamula, Rediho etc. All you need to do is show up with your gear. They practice buddy diving and may want you to do an orientation dive first - usually at the excellent site just off their dock known as either Playa Kalki or Alice in Wonderland - depends who you ask. It's really a great shore dive anyway and their dive dock takes you 50' closer to the reef. We dove it twice - south is just a good dive. North snorkel over the shallows till you're past the condos on the cliff (the Lodge KH condos) and look for flounder. That dive is more vertical in some areas.
The standard suggestion for anyone staying that far out in a condo is to buy groceries in town before you drive out. Centrum Market is about 2 miles south of the airport - go south thru the rotary instead of west to Westpunt and it's about 2-3 stoplights down on the right. They even sell $1 used styro packing crates and IIRC dry-ice for the 45+ min. drive out. Cost-U-Less is also in that area but I'm not as positive about where.
You've got it right that most of the shore dives between Westpunt and Willemstad have on-site faciliities - usually a dive shop, food option etc. Most are also beach entries - if they need a dive dock one has been built by the operator. Most will watch your car if asked, we locked it and kept stuff inside all week. Curacao Tourism often has a guard at Porto Mari watching the parking etc. - a good locals beach plus easy entry and very good double reef dive. Porto Mari Sports is on-site.
Where are your friends getting married? If it's any of the major resorts in town they probably have a shop onsite or affiliated. So that could be another option.
A couple other dive things: the Dive Bus is located near Pierbaai Beach (south Willemstad but pickup most places in town as far north as the Hilton in Piscadera - IDK their south limit. Bas Harts also guides privately in the area. And Goby Divers at the Veneto alao picks up locally - map is on their website. Last but not least, Relaxed Guide Dives offers tank rental packages - they're located just west of the Airport on the main road. Staying out near Westpunt they wouldn't be as convenient but if you stay in town and drive west to dive, you'll go by it every time. We probably did a dozen times in 10 days.
Curacao being a long island you have to be a little selective in dive boat choice - most of the west ops don't go as far east as town and almost none of the town-based ops go as far west as Watamula - or even Musthroom Forest. Check their dive site lists - sometimes it's easier to pick a couple and drive between them - we did. Dove with a now closed operator west and dove east with Ocean Encounters main shop across from the Sea Aquarium. They do the Dolphin Dives if that's of interest but it's + $200 pp. now afaik.
My take on food prices - I bought for 5 at Centrum b4 driving out - it was expensive but not unexpected. I don't remember dining out to be that high - most of the beachfront places are pretty casual. We liked Ribs Factory in Salinja once - prices were the same as a steakhouse here.
The Marriott's beachfront restaurant was a little pricey but you're paying for the table in the sand 40' from the water. There's more affordable options - once we ate breakfast at Denny's in Piscadera. Most lunches in dive affiliated locations were under $10. Two big lobsters, potato etc. was $27 at Surf/Turf on the east side - probably less than Red Lobster. And I'm pretty sure the cooks helper walked down to the beach and got them out of a trap so they were fresh killed.
