Just before Habarana town when you are heading from Dambulla.
Hannapier, Ritz and Franklin are the known brands to me.
Yes 3 nights needed in Sigiriya/Dambula and may even 3 nights to Kandy if you can.
Put A'peak between Ella and Kandy as it is the better order of travelling
Since it is a Van to accommodate four of you, 85 is not that high as I see it. To reduce the cost just do the sightseeing and release the driver after.
Mix Dettol liquid with any sort of oil (Coconut,Veg,Olive etc) and apply it on your legs and arms. This would keep the leeches away for a while.
Yes as said Wilapattu is only enjoyable in a full day safari or you can book a wild life dept bungalow ( only basic facilities as far as I know) inside the park...
Jan 1st NY day or the Sri Lankan NY day which is APL 12 and 13
Since you have a month you can go off the beaten path and visit other places such as Mannar, Wilpattu NP, Mauduru Oya NP, few ancient temples off the path.
D5 seems too packed
May be you can decide on two bases at the beaches, like Unwatuna and Tangalle or Mirissa. Even I think Yala beach is a good option