When it comes to epub3, I think you're simply going to want to wait on the next version of Sigil to do much of anything. It will be more robust in its abilities to recognize and edit epub3s. Your error indeed stems from the fact that you've instructed Sigil NOT to fix a parsing error it could very easily fix if you allowed it to.
But you're not entirely wrong to turn off the "Mend" feature (for editing Epub3s you've edited with calibre's editor) in Sigil 0.9.2, either. calibre's editor doesn't use (and will often remove) doctypes. Sigil 0.9.2 will assume a document without a doctype is xhtml 4.01. That will definitely break a valid epub3 that doesn't contain doctypes if you allow it (Sigil) to Mend code.
Rest assured that Sigil 0.9.3 will be able to handle this type of situation (IFyou turn the "Mend" feature on and allow it to do its thing). 0.9.3 uses the opf package version to handle doctype decisions now. It won't break epub3s coming from calibre's editor with no doctypes. But you WILL have to turn on "Mend", at that time, if you want Sigil to be more tolerant of simple parsing errors. It can't fix errors you instruct it not to fix. Mend is what
makes Sigil more tolerant of simp!e parsing errors. Turning it off is instructing Sigil to be intolerant..
I hope that explains things. The only "real" danger in leaving Mend Code turned on in Sigil 0.9.2 is when opening Epub3s with no doctype. And that's fixed in 0.9.3. You will also be able do a lot more editing of epub3s with 0.9.3. 0.9.2 is very limited in what it can do with epub3s Admittedly so.
Thanks for the sample. We'll check to see if Sigil's xml/opf parsing routines can accomodate a syntax error of that nature. Just understand that Sigil is going to be less forgiving of syntax errors in an opf file than it it is of markup in an html file. The OPF is, after all, the beating heart of an epub. A lot depends on it being right.