Ok thanks to all. I consider all problems now solved.
1) I needed to reinstall Java 8 by downloading from Java website because it wouldn't update. I did not need to remove Java first.
2) I needed to download new Start.jnlp from U-D forum. Since the icon it makes (when cache cleared) depends on a later macOS version, I could get admin.jnlp instead for a faster startup (but no big deal IMO).
3) I needed to click the lock on Preferences Security page to run Start.jnlp the first time.
4) There is no problem with 4.9.0, the version of Firmware and UI that I already had. In fact it's the last version compatible with the earliest Zwave modules (300 series) if I happen to find one of those. Otherwise I'm going to wait until I can buy the next U-D package that includes Zwave or other protocol that I decide I'm going to migrate to.
In the meantime the price of used Insteon modules on ebay has fallen from ridiculous scalping to normal or less.
