It might be a little late for recommendations, but I thought I'd still give you some tips about Maui! My mother's family is originally from Oahu, but my Aunt currently lives in Kihei in Maui and my fiance and I have visited her there twice. We actually recently got engaged on our trip to the island just last month! Like you, I'm planning a wedding in Maui and have been dealing with a lot of the same questions and concerns you have. But, I will say I that I wholeheartedly agree with choeft201 that South Maui is definitely the way to go! You're right that Wailea is more expensive, but Makena and Kihei have much more affordable options for hotel accommodations. As of right now, my guests and I are likely staying at the Aston Maui Hill in Kihei which has fantastic pricing for condos, but any of the Aston resorts are a good value and are really well-managed! The problem with West Maui (Lahaina and Ka'anapali) is that you're more likely to encounter days of cloudy or rainy weather because of the mountains and wind on that side of the island. Also, the beaches in my opinion are more crowded (due to all the resorts), not as pretty, and very windy. So, although there are resorts and hotels a plenty, South Maui actually is the better place to stay (and get married for that matter) if you're worried about weather because it rarely gets rain or prolonged cloud cover.
Also, the nice thing about South Maui is there is SOO many things for your guests to enjoy! Kihei has great beach bars, casual restaurants and shopping and it's only a 5 minute drive to the absolutely stunning beaches of Wailea and Makena. Makena Beach (or Big Beach) is rated the most beautiful beach on Maui and they aren't lying! Makena is relatively undeveloped so there are times where you can have a beach all to yourselves!
Also, since renting a car on Maui is a MUST (no reliable public transportation on Maui), you could still travel up to Lahaina and it's only a 35 minute drive. It may sound like a trek if you're used to staying on Oahu, but the drive is stress-free and the scenery is so beautiful you'll forget you're driving! For people who stay in Ka'anapali, the drive is pretty long to get to all the beautiful locations in South Maui, so they end up missing the best of what the island has to offer! Other than it's proximity to Lahaina, the Ka'anapali area doesn't have as much to offer as Kihei/Wailea/Makena in terms of beaches, nightlife, shopping, and restaurants, so it was definitely the first on my list for places to recommend to you! It's important to balance your pros and cons and every group's needs are different. But, in my opinion, the pros of staying in South Maui and having a wedding there far outweigh any negatives. Hope this helps!
As for group activities or excursions, we have thought about chartering a boat to take us to Molokini. The only downside is that if you get a cloudy day, snorkeling is not nearly as amazing as everyone makes it out to be because the colors don't really pop without sunshine! One thing my wedding party and I are going to do when we go next summer is the Old Lahaina Luau. It is by far the most amazing show I've seen in Maui! My aunt also recommended seeing Ulalena but we have not done that yet. Other than that, almost all the bars in Kihei are within walking distance of each other, meaning you can park at one of the Kamaole beaches and have a night out! Most also have nice views of the sunset, Moose McGullicuddy's in particular, and great happy hour specials!