>We like some quiet, but want to be accessible to town and have comfort with some economy.<
Any of your choices should work for you fine - each one has a little different personality. We began staying at Caribbean Villas when it first opened. Caribbean Villas is quiet, with only 10 rooms/suites on 3 acres of land. They are about 3/4 mile south of town - a very nice 12-15ish minute beachwalk - fun and safe walk, night or day. Caribbean Villas has both rooms and suites so you have reasonable price choices. They do not accept commissions from guides or tour companies, so you receive objective recommendations and bookings for excursions and in some cases, lower prices. Caribbean Villas has free bikes, hot tubs, gardens, wooded bird sanctuary, 300+ foot pier with swimming and snorkeling area. The staff has been there long-term, are great people and they love guests. When you arrive on the island, you will be picked up at the San Pedro airstrip in a golf cart and provided a town tour on your way to your room or suite.
>Any recommendations of a kind and excellent PADI instructor?<
Amigos del Mar is an excellent dive shop with a fine instructor. Recommend you contact them through their site via "Goods & Services" drop-down menu above. You might ask them about you completing the classroom portion of PADI in the US and then finishing the certification with dives through them. Your certification dives would still be scenic. AdM has an arrangement with Caribbean Villas - ask them the costs of dives if you are staying at Caribbean Villas. They will take excellent care of you as a new diver and you will feel like a pro after the trip.
>Finally, air taxi or plane from Belize City?<
Recommend you fly Tropic Air or Maya Island Air from Goldston International to San Pedro. Only a 13-15 minute flight and much easier than other options. Flights leave every hour.
Happy first visit!!