It's not been added by us on recent versions because it was in beta and causing issues with other mods. You can find them and use them at your own risk here: https://www.curseforge.com/minecraft/mc-mods/multiconnect/files
Actually.. looking further into this, IPN has a couple of dependency mods, libIPN and Kotlin for Forge. After adding these required files it does not crash with or without Aristois
Essentials is a plugin. Are you referring to "ForgeEssentials"? Or Bssentials?
The addons marketplace is only for Fabric, not Forge
The marketplace has FabricAPI, which would imply you're not running forge, and ForgeEssentials is a Forge mod.
The installer currently can't be used on Mac, due to how MultiMC is an app instead of a folder. You can use the zip file to install it.
Visit https:/maven.aristois.net/manifest/(MC VER).zip?launcher=multimc, but replace (MC VER) with the Minecraft version. It will download a zip file containing the instance (Ex: 1.19.3-Aristois). Drag that folder into your instance folder, then the instance will appear and you can use it.