I pretty much agree with the above^.
One thing about your poll to note is that on Lana'i you pretty much have to be Four Seasons folks to stay there. Also there's one dive operator - Trilogy Water Sports - and only one shore dive at Hulopoe Bay - the rest of the island is too tall. Trilogy prices reflect that they work mostly with the Four Seasons guests also - they're in the $175 range compared to the day boats from Lahaina who do the same dives from Maui for $129. There's a lot of nice coral off the west side of Lana'i. And certain areas near Molokini also. Both easily accessible on morning trips from Maui.
The best shore diving is off Maui also IMO. Entries are certainly the easiest, many off the beaches. In the Kihei/Wailea area there's about a dozen dive sites within a 5-10min. drive. I like that area best as Wailea is a few large resorts interspersed with condo complexes - some waterfront. Up around Ka'anapali and Kahana you get a lot more of the mega-condo towers dotting the beach. Napili is pretty nice also - Kapalua is more upscale.
Sites on the Big Island are significantly more spread out so there's a lot more driving between the best of them. Most of the boats moor in Honokohau harbor just north of Kona so if you go there - probably finding something just south of Kona is your best option. Besides all the big resorts there's a lot of affordable condos on sites like vrbo.com.
The not to miss dive off Kona is the Manta Night Dive. It regularly makes the top 100 dives in the world lists.
All I know about Molokai is that the only operator there seems to be Scuba Dive Molokai - which is Molokai Dive & Fish's dive business.
Even though the list I linked to below shows more, there's really only two good shore dives off Kauai - Koloa Landing and Tunnels Beach. The rest are mostly too shallow or closed. One is north and one south so plan on an hour to drive between them. We tried Lydgate, it was too shallow. And Ahukini is closed to divers now. fwiw, Kauai is probably the most scenic of the islands, certainly the most green. All that active volcano stuff makes the Big Island look bleak by comparison.
At your dive count, Ni'ihau is probably not the best idea. It's also a deep wall dive where the thrill is the reef sharks and the occasional monk seal. Google Vertical Awareness to see one of the famous dive sites there. It's also an all day, rough crossing from Kauai - only done in spring/summer.
This might be useful:
Scuba Shore Diving Region: Hawaiian Islands
I voted Maui - for first timers it's IMO the best overall Hawaiian experience. Unless you want to see Pearl Harbor and Waikiki Beach. Or Volcanoe's National Park - which I found underwhelming from the viewpoints.