Hi'
I'm in Sri Lanka for two weeks right now. Just got back from Unawatuna which was fantastic. It has a village feel as you can walk around all the shops, beach, restaurants, hotels within a few minutes. There are enough people for an atmosphere, but not too many. Youj can dive there, nothing world class, but there are two wrecks, I saw one called Tango wreck, which was nice, but would only satisfy one dive. There are some caves to dive apparently. There's probably 2-3 days of diving if you exhaust it, but that's a guess.
I stayed at Thambopanni which is the inner land location of Thrabopan. Both are very nice, and $45 a night is cheap by european standards, but I guess probably more than the rest in Una. It is very nice and chilled out, good service too, so if the price isn't an issue ten its a good option. I didn't see anything else that I thought I would check out and recommend. There is enough availability in the area to turn up with nothing planned, but Thambo and Thrabo. will get booked up. They have a good website which isn't too deceiving. The Thrabo is on the beach front, thamopanni isn't, but we didn't mind because then you can go to the restaurant in Thrabopan and sit by the sea when you eat, and restreat to your accommodation afterwards.
I haven't seen the other places you list. I hear Hikkuduwa is not as ncie as there are a lot more people. I would definitely recommend Una! Enjoy!