Bruce:
It's not a condo, but maybe check out Hamanasi Adventure Resort. They're on the mainland, Stann Creek area just south of Hopkins. Dive op will go out for 2-tank barrier reef or longer 3 tank atoll trips. They also have their own tour guides and vans and can get you to the "ATM" (Actun Tulum Matal or something like that) wet cave / burial site (well worth it! 1.5 hrs each way of ankle to chest-deep caving with actual Mayan relics and heavily rock-encrusted skeletons at the end), Jaguar preserve hikes, ziplining/river tubing, the Xuantunich (sp?) and Cayal Peche Mayan ruins near the Guatemala border, etc. There's also more local options like just toodling around kayaking along the more local river mouth or birding in the morning. They've also got their own beach and pool (but 'ware the sand flies), no real snorkeling, although they will take snorkelers out to the barrier reef as well. Basically you buy a package that gives you X nights, meals, and Y excusions, and they have signup sheets and do them all on a rotating schedule. With a few exceptions (whale shark trip, further atoll dives) they go out without any minimums. Only a few of the excursions are added cost (ATM being one).
Food and service at the resort are spectacular; I cannot say too many nice things about them in that regard. Rooms are very comfortable. If you wanted to go and just dive-dive-dive not sure I'd recommend them, as they don't do as much diving (schedule doesn't really allow for 4 dives a day, in general). But as you said only half the group dives, it might be a good compromise. (That's why we picked them - wife wanted a more mixed vacation and I surrendered vs. staying on one of the kays directly.) Local town is pretty safe if the teens are reasonably steady and not looking for trouble, if they wanted to wander around Hopkins. (Resort also has bikes.)
Barrier reef dives were say 30-40 min boat rides there and back, but it goes pretty quick. Atolls were more like 1 1/4 to 2 hrs each way. We only got to do one due to weather (we arrived right after that TS passed right over Belize City last year so the chop was contrary to normal and the water was kind of silted up - in fact, we were on the flight that tried to land during the height of the TS, but the captain bailed on final approach and put us down in Cancun instead...only to have to fly us back to Dallas that night, costing us the first day of our trip.) You're not going to get much shorter than those though unless you stay on one of them, and then you're dealing with the mainland trips for everything else. Much more pleasant to sit on a boat before/after a dive vs. before/after a 4-5 hr hike, IMO.
If you stay ANYWHERE on the mainland, one suggestion: find that 100% DEET stuff that comes in the tiny tubes, and buy two. Per person. The typical 15-20% "Cutter" or whatnot stuff just doesn't cut it, at all. (Another reason that ATM cave hike was so great - once you hit the cave mouth and are in cool dark water, it's amazingly soothing on all the bites. 
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