A standard feature of dropdown list controls is that you actually can type in them. I think it works in most software, not just Construct. It's kind of an invisible feature so is not very discoverable, but providing the dropdown is focused, you can just start typing and it will select the first item that starts with what you typed. You can start again by just waiting a short period of time which resets it, and start typing something new again.
Also if you have loads of global variables, maybe you could use some other techniques to better organise them, like local static variables, use global Sprite instances with instance variables, use a Dictionary, etc.