I tried to use the search feature and can't find a thread on this, and if there is one, it's possibly old enough to be lost in the noise of other posts.

Right now, mirc will indicate when you use a color code, bold, reverse and strip codes. As for italics and underlining, it just inserts a space... sometimes. I often find myself guessing if I've properly underlined something, and am unable to determine it unless I hit enter to post it.

This is whether it be for a script or for just chatting. I guess overall I'd like a uniform setup for the codes so that I know they're working properly, instead of wishing for the best of luck.

This is especially annoying if you're trying to edit a topic, since it definitely doesn't insert a space when you're in the channel manager dialog window.

Also, it seems the symbols that DO pop up for bold and color changing have weird spacing issues. Is there a possible fix to that?

Pretty sure that's because the font you're using doesn't have printable symbols for those characters.
Try this one http://www.xise.nl/mirc/fsex2p00se3.ttf

That font still seems to be lacking the italics and underline symbols.

That has been suggested in the past but ascii 0-31 are called non-printable characters, it makes sense fonts do not render them with a visible glyph.
Yet they do, the problem is now that you're unhappy with the glyphs they use (or at least for those two control codes).
And since mIRC lets windows handle the way the characters are displayed in editboxes, it's' more or less a problem depending on the font used. Note how you're not reporting the problem for others control codes because you're fine with how they are rendered.
I agree though, it would be better if control codes were rendered with consistency regardless of the font

I just remember back before Windows XP that when you used codes, you'd get [] boxes regardless of font.. but if you installed foreign languages, it became the weird symbols I mention now. In windows 7, the symbols just seem to be the norm. I kinda wish the []'s would come back somehow.

This broke in Vista and it's been raised as an issue before but I'm not really sure what can be done... Windows refuses to display these characters the way it used to. Note that you can use custom fonts that DO have glyphs for the non-printables, like Fixedsys Excelsior (specifically the version linked above).