Secondly, while such interspecies beings (mangels?) could exist, do the Fathers or other saints talk about, or maybe describe them?
I've not looked into any post-St Augustine saints' commentaries on the Nephilim. Mostly because, as is normal, they would've adopted +Augustine's opinion and added no new info. That's why Anne Catherine Emmerich's comments are so unusual - she both agrees (mentions possession) and disagrees with St Augustine (mentions fallen angels and Enoch).
My opinion is
a) there were angels who fell during Noah's time (too much evidence for it, both from Enoch, Church Fathers and pagan civilizations).
b) possibly the "genetic material" for such Giants was 100% human, but enhanced/altered due to fallen angel possession/knowledge. This would agree with St Augustine, Scripture, Enoch, Anne Catherine Emmerich, Ancient Aliens and the great X-files tv show, etc. This theory agrees with all sources because Scripture does not say "how" such Giants were fathered, only that fallen angels were part of it.
c) Such a theory also coincides with the future anti-christ, who will be 100% human.
And the reason this is all important? Because Christ said "As in the days of Noah, so shall the coming of the Son of Man be". Obviously, Christ was first talking about sin and lack of faith/religion but I also believe that we will see "signs and wonders" in the last days and also a return of giants/fallen angels/"alien" demons/etc. Imagine what kind of technology "bump" our society could have, if fallen angels were directing the elites today? Lots of people in govt say that "Area 51" is all about super-technology and that it's 50-60 years ahead of what we see now. I think the "alien visitors" story will happen because it's the only way for the elites to introduce society to fallen angels, and a "new world" of futuristic technology. In the next 10-20 years, I expect this to happen, if God doesn't intervene.