The eMMC is not very durable so it will fail earlier than an SSD. Which I think you already know.
As far as I know, this is not the case. If your OS drive does you lose your config with it. You don’t lose any data, but you will have to resetup truenas from scratch.
You would need the config from the eMMC to carry on like normal. In order to backup your config there are a couple of ways to do it. The official way is to use TrueCommand. The perhaps better solution for most people running 1 machine at home is to use Multi-Report written by joeschmuck. It has the ability to email you your config so you can reinstall if needed.