In v19a doesn't work for anything, mainly because it has some typos like this:

[\\{\\[]]

The double ]'s at the end there mean it tries to match a literal ]. As-is, v19a won't match practically any var. Beyond that, what you've shown here also does not work on numerous lines because of what i mentioned before, it wants every line to not contain { or }  and so it breaks on nested dictionaries.  The second half of the major | also is needlessly limiting what it can match on. It should just match everything up to the newline, otherwise it breaks on numerous valid lines.

The regex I've proposed works much better on a wide variety of use cases, the regex101 link loads it up in the unescaped form and includes several test case matches, which your proposed one, and past ones, fail on. For example, this has never worked as a new variable to insert:

var randomportraits = load(globals.modfolder + "/randomportraits/randomportraits.gd").new()

Prior regex's would always chop off part of that, then insert an invalid statement into the scripts.

Plus the later code when using AddTo was stripping {}'s which also prevented any insertion in nested dictionaries.  With the proposed modpanel.gd changes I made from v19a though, I was able to successfully test a mod to races dictionary. Should also work for multiline arrays too.

I'd recommend for future releases you make a test mod that tries to insert new and modify existing entries in several methods and make sure that #1, you can apply it, and #2 after applying the game starts without errors.

Edit: Just saw your edit3, that still does not work.  Did you try plugging that into the regex101 link I gave? That includes a bunch of test cases, and it fails on the multi-line.  Is there something wrong with the one I've proposed? I've tested it against a wide variety of test cases and successfully used it in-game.

Iit could be more robust, but at a certain point working with regex's is infuriating... I can think of more valid gdscript syntax this wouldn't currently handle, but shouldn't be the mod system's job to handle everything, only the sane things, like all multilines must be in form:
var blah = {
}

or the same with []'s, and not allow inserting anything extra after the starting or ending []{} characters.  I'm sure at some point someone will write this in a mod though and bitch it doesn't work...
var fizz = { buzz = [
"moo"
]}

What I've proposed though should work with modifying any of the existing code's current bits as they always follow that good standard as far as I've seen.

Though realized one thing I did not update.  I made AddTo work, but did not test RemoveFrom. Best guess, it works but maybe not as expected because the first line will be considered to be the { or [.