I just downloaded RetroArch this weekend. The startup is confusing at first, but this guide helped with the basics: https://lifehacker.com/how-to-set-up-an-all-in-one-retro-game-emulator-with-re-1745863517. While running RetroArch, select online updater, and then select the gaming system that you want to "update," i.e. download. RetroArch will then install the latest versions of the supported emulators that you select, which are referred to as "cores." Genesis Plus Gx and Picodrive are the two genesis options. Both have been updated recently. To start a game you have to "load core" (emulator), and then "load content" (rom) by accessing the directory where your rom is stored.
Netplay is intriguing because it supposedly uses a peer-to-peer setup with frame rollback similar to ggpo to reduce latency.
Here is more in-depth info on setting up netplay. Bottom line, it should be compatible with Hamachi. https://forums.libretro.com/t/looking-for-comprehensive-guide-on-netplay-willing-to-make-one-myself-if-none-exist/4523