Isn't is simply possible that on an alien planet with unique atmospheric conditions that fossilization can take place within its own set of physics? Why are we obsessing over what stage of fossilization Dallas claimed he saw on LV-426 --- what if Dallas was completely wrong? What credentials did he have for making that assumption = fact?
The murals are the key -- they are ancient. The Aliens existed for a looooong time as did the Engineers. Occam's Razor my friends.
Furthermore -- in my humble experience on planet earth and in internet forums dedicated to movie franchises -- 98% of all fan theories turn out to be false.
Of course we all have a choice as to what we want to fit in our "head canon" -- Star Wars less so now that Disney rebooted it -- but Alien / Pred
There is no reason why, if you don't like whatever turns out to be new in Covenant that you can't simply pretend you died last year and the new mythos additions never existed for you.
My guess -- considering the ancient nature of the Engineers and the Goo -- Aliens of all types exist throughout the cosmos.
All David will end up doing is adding the "Mech" to the Bio-Mech. Not creating the Alien from scratch. This actually adds more gravitas [in my opinion] to Shaw wanting to meet her maker. It's a vicious circle of life where makers and "mades" are yinning and yanging from Engineers to Humans / Spores / Aliens to Androids to New Breeds of Aliens who destroy ecosystems until Predators come and wipe them out in order for Engineers to re-seed life on that planet. Easy-peasy, no?
And if, for some odd reason, you are so put off by David having such a major role in things of this nature -- read this: Elon Musk doesn't think it's a stretch at all and even see AI being this ingenious as inevitable.
http://waitbutwhy.com/2015/01/artificial-intelligence-revolution-1.html