So I'm in the latest version of Zerbu's mod constructor but it won't allow me to export a package because of the custom tuning with my "Blow Sim Up" interaction which I hope works. It starts with the Sim (Killer Queen) giving an innocent hug to a Sim and then the receiver of the hug explodes and dies.

However, it won't let me make a package file, stating "Unexpected XML declaration. The XML declaration must be the first node in the document, and no white space characters are allowed to appear before it. Line 1, position 9." But when I look at my XML declaration it looks fine to me. It gives me this error message even when I remove all my attempts at custom tuning. Here is the XML in question, which is the only one I've tried to customize.

https://pastebin.com/nA9wP8h9

Not sure if you've figured this out already, but in case you haven't... I don't have a lot of experience with Zerbu's mod constructor, but it looks to me like there are multiple file definitions in that one XML? And the first one is incomplete. Lines 1-52 is a SocialSuperInteraction, but many of the tags at the end are not closed. And then there's a declaration of a DeathSuperInteraction on line 53. Fixing those two things should get you closer, and if you keep running into issues, try using an XML validator (can literally google XML validator). Something that will help in the future is to maintain consistent indentation; keeping start tags and closing tags at the same indentation level makes it easier to tell what tags are closed and which ones aren't. There are also tools online to help with this called XML prettifiers. I'd look into those as well.