backwards conlanging

Query:

I've read several conlang sites that suggest starting with an
ancestor-language and then deriving conlangs from the ancestor-conlang.

This is a good idea.  The problem (for me) is that I started developing
Chevraqis before I'd read about this (or it had penetrated, anyway).
Since then I've been struggling with the ancestor-conlang trying to get
the thing to come out so I don't shriek in disgust whenever I look at it.

Can you work backward from a single conlang to develop an ancestor-lang?
I figure I could do that with what I have, reverse-engineer the ancestor,
adn then resume "normal" conlang production by creating stuff in the
ancestor and evolving it to the current language.

Crowley's _Historical Linguistics_ explains about reconstruction of
ancestor tongues, but this requires taking actual languages that really
did probably evolve from a common ancestor and working backward from
several of them, if I understood that chapter remotely correctly.  I
don't want to just randomly create another language and try to work
backwards, because in that case they *won't* have a common ancestor like
in "real life."

Does anyone have suggestions for a related-but-faux conlang technique for
same?

YHL, who is otherwise ready to scrap the entire mess and start from
scratch, darnit