Proof is in having the drafts as well the as final product.
I am very familiar with Australian, British and US copyright law - have been since the mid 70's when it became an issue in my awakening and burgeoning interest in anything to do with being successful in the music industry. In those days you had to find the right books and read them - really read them, rather than skim some bs website that was written by a 14 year old (or someone with the intellectual capacity thereof) about what they think the business is about.
The "Proof of ownership" is a legal fraud perpetrated by the US legal system in attempt to cow people into complying with dominant business models instead of recognising independent producers and writers. Copyright exists in any composed work whether that is a book, a poem, a lyric, a song, a piece of instrumental music, a film, an article, a website with original content from the moment the idea is committed to any form of media: pen and paper, computer disk, audio cd, film, acetate, printed. Ownership beyond this is an issue for the courts.
I still believe that you are being overly precious and pretentious when it comes to demonstrating your skills and your ish.