It is possible to get an instance list of its variables
Well, in short, no. Not unless you make your own script for manually drawing all the required variables.
and if not what else could I try to fine tune variables in-game?
What I use for custom debugging / testing / tweaking during runtime is my own String Parser, combined with get_string() function. It's not as versatile as it would be if Game Maker was still an interpreted language, but it's as close as you can get in a compiled language...