If you can accept a couple of days in a non-luxury place, the hotel Eva Lanka is very close to Amanwella however is on the nicer beach which is calmer. This being said- it is a 3.5 star property although run by a nice Italian/Sri Lankan family- when we want to be on the beach here this is where we stay and it is OK in terms of property if rustic with the beach a 10/10.

I am not sure what you mean about cross-country treks? The drive is 90 minutes from Galle to the area where Eva Lanka and Amanwella are located. The last two years in a row we went swimming at the Eva Lanka in December so it is doable without any real troubles and a bit of common sense. If driving is really not your thing we did meet some people who had chartered a helicopter which you should be able to set up easily through the Aman hotels (there is a group out of Colombo that run both seaplane and helicopter charters all over the island).

Serene Pavillions are 30 km from Colombo- so you won't get to see a lot of the country if you end up staying there- at this point I would head over to the place in Bentota run by the Paradise Road Group which is at least a little closer down the coast (closer to Galle).