Lua API unload luaL_loadfile - api

Question

I working on Lua scripter in game that users are able to write custom Lua script and load it.
I already done with implementation and functions
L = luaL_newstate()
luaL_openlibs()
lua_register(L, cFunction, "luaFunction")
lua_register(L, cFunction2, "luaFunction2")
lua_register(L, cFunctionN, "luaFunctionN")
What I trying now?
Possibility to execute/kill script on button press, below picture for more clarity : https://i.stack.imgur.com/eJQzg.png
All scripts from scripter need to access to Lib.lua so i do:
luaL_loadfile(L, "Lib.lua")
lua_pcall(L, 0, 0, 0)
Then to load script i could use this same thing and is ok until user want to kill/unload script.
luaL_loadfile(L, "script.lua")
lua_pcall(L, 0, 0, 0)
I digging in many threads about Lua API and currently no good answer for this problem.
Ppls talking about lua_newthread that i already tried to implement but no success.
T = lua_newthread(L)
luaL_loadfile(L, "script.lua")
lua_pcall(L, 0, 0, 0)
Return >> Attempt to call a nill value. Look like new thread has no access to Lib.lua
Also there is other problem, when i register function i use:
lua_register(L, cFunction, "luaFunction")
Then even i able to create a thread functions from luaL_newstate*L required to pass handle L to working while handle of thread is T.
Example of C function passing to lua:
static int add (lua_State *L) {
double a = lua_tonumber(L, -1); /* get argument 1 */
double b = lua_tonumber(L, -2); /* get argument 2 */
lua_pushnumber(L, a+b); /* push result */
return 1; /* number of results */
}
Any king of is appreciated.
Regards,
Ascer