I don't know how essential having a beach immediately accessible is, but if you want romantic and stunning (and much closer proximity to the volcano) consider the Palms Cliff House on the east side of the Big Island near Honomu and north of Hilo a bit.
www.palmscliffhouse. It is exceedingly pricey and exceedingly romantic
There are no white sand beaches on the east side as there are on the west side (you'd be talking an hour and a half drive to Hapuna or Spencer beaches). But what the east side does have is so much more "old Hawai`i" than the west side and much less of a touristy experience. There are a few very good restaurants (e.g., The Seaside, Cafe Pesto, Hilo Bay Cafe, Coconut Grille) in Hilo (and Waimea is an hour away -- where, imo, the best restaurant on the island is). You're an hour from Volcanoes National Park (versus the > 2 1/2 hours from the Kohala resorts area.
I suspect, knowing both the owners and the cook at the Palms Cliff House, that they would try to help with any dietary stuff if at all possible.