You can't spam the SavedVars anyways. They usually are written when you reload the UI or log out. There was also some kind of autosave system, saving once every 15 minutes with 1 manually triggered save allowed (but no idea how that works).
In general if you send too many requests to the server in a short amount of time or if you run a local calculation that freezes the main thread for too long, you just get disconnected. So basically you can't really break anything by mistake. But it never hurts to use the API in a sensible way and not overwrite global values and so on.
Regarding this "Lua thing": If you want some auto completion I can recommend using LuaLS (formerly known as sumneko.lua) as a language server. You can reference ESOAPI definition files and files from other AddOns as external libraries. Also Lua5.1 != ESOLua. Check
https://wiki.esoui.com/Esolua for details. PS: The mentioned Coroutines don't work yet.
If you want validation for XML-GUI stuff, sirinsidiator made definitions that you can include:
XML Code:
<?xml version="1.0" encoding="utf-8"?>
<GuiXml xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://sir.insidi.at/or/schema/esoui.xsd">
...
</GuiXml>
Also check out
https://www.esoui.com/forums/showthread.php?t=9867 if you haven't already
Have fun with your addon development.