I have been to Hvar island several times over the past few summers and have previously always taken the ferry and rented a car at Split airport. I am going again in September and I too have been looking into the seaplane. I always stay in the north of the island rather than in Hvar town, which is easier for the ferry and the seaplane.
Seaplane - the total checked in weight is 15 kg and there is a size restriction of 140 cm of the total dimensions of the checked in baggage (length plus depth plus width). In addition you can carry on a piece of hand luggage with a max weight of 5 kg.
The seaplane lands at Jelsa in the north of the island. There are few buses on the island, but one always meets the ferry to go to Hvar town. I am not sure about conections to the Jelsa seaplane, but looking at the online fares, the airline seems to offer fares to Jelsa or to Hvar. For instance going tomorrow, there is a 17 minute wait in Jelsa for the bus and if this is missed, the wait is more than 7 hours! Hvar town is c 30-40 minutes drive from Jelsa. Jelsa, Vrboska and particularly Stari Grad are charming villages and should not be overlooked. The last twice I have been, we have not bothered going to Hvar town, as it is over-run with loud partying yachties in the summer months and anyone over 25 feels out of place! It is a charming town off-season.
There is pay parking at Split airport and plenty of it - I am not sure of the cost but I am sure you can find this online.