Can't get MarioKart Wii Backup to start!

[AnyOldName3]

Nope - master branch dolphin has had netplay for years. The netplay branch (which stopped with 652) was created to test experimental options for it before a proper implementation was added to the master branch. The main two were dual-core netplay (which is called Deterministic Dual-Core in master as it's a more general solution) which was added several months ago and UDP netplay, which was added more recently; a couple of months ago. Single core netplay worked fine, and was actually faster than 652's dual-core right before the DDC merge anyway, so there wasn't really any excuse to use that as justification to use 652, and so did direct IP connection netplay, which requires you to forward one port on the host machine's router and share its IP address with all the clients.