Sounds like an amazing trip!
I just came back from 7 days in Kona. I was there during the initial eruption and 6.9 earthquakes. Kona is fine and the diving is still great. There are higher levels of VOG in Kona so it is overcast. I recommend diving with KDC. They have two boats now and are doing 2 tank local dives on the bigger boat and 2 dive long range dives on the smaller boat. In addition to the 2-tank twilight/manta dive they are also now offering night reef dives which are amazing! I still need to write up a trip report, but in my nine dives, I saw lots of juvenile schooling fish, octopus, squid, spotted eagle rays, tiger sharks, pipefish, turtles and even a mantis shrimp.
I was fine in Kona but it could be tough if you have asthma or other challenges. One other option is to stay and dive in Kohala with Kohala Divers.
My understanding is that Volcanoes National Park is mostly closed because of air quality issues. There have been some evacuations for the lava flow and I think about 25-30 homes have been destroyed, but most of the evacuations are in Puna and are related to air quality. The area being hit with lava is called the rift zone for a reason -- there were evacuations in 2014 too. The big concern now is a steam explosion--which is why I wouldn't recommend staying in Kapoho, Pahoa, or greater Puna right now. But the rest of the Island is open and safe. I'm trying to head back in late June.