How to load a preset / LrTemplate file into a table variable

Call loadstring() on the string return by readFile(). That will compile the contents of the string and return a function, which when invoked will execute the Lua statements in the string.

LR uses the convention that its template files are Lua statements assigning to the global variable "s".  So after you execute the template file, the global "s" will be assigned to the table stored in the template.  Assigning to a global variable "s" isn't very polite, but that could be made completely safe by using Lua's setfenv(); but unfortunately, LR doesn't provide setfenv().  As a practical matter, copying the value of "s" immediately after executing the loadstring() function is safe enough, since the task scheduler is non-preemptive.

If you really wanted to be compulsive, you could strip off the "s = " and replace it with "return ". Then when you executed the loadstring() function, it would return the value in the string.  Some of my plugins store Lua constant expressions (e.g. tables) in files, and it uses this method to read them.  E.g.

--[[----------------------------------------------------------------------------
public any value, string errorMsg
stringToValue (string s)
Executes the string "s" (created by valueToString (v)) to return a value
equivalent to "v".  If no error occurs, returns <value, nil>; otherwise
returns <nil, errorMsg>.
------------------------------------------------------------------------------]]
function Util.stringToValue (s)
    local chunk, errorMsg = loadstring ("return " .. s)
    if not chunk then return nil, errorMsg end
    local success, v = pcall (chunk)
    if not success then return nil, v end
    return v, nil
    end