The original Tokyo Ghoul adaption is perfectly fine, you monsters! If you want to see a true abomination, by the very definition of the word, look no further than :re.
So even other writers/artists recycle the Saber design. The first thought that comes to mind is 'unoriginal', but I suppose there could be some deep, thought-provoking explanation waiting to be revealed. Multiverse shenanigans are always fun. Sometimes hard to sell. Which is probably why Disney went "no thanks" when they got their hands on Star Wars.No, El-Melloi is done by someone else.
Not an expert on Fate, but as far as I know, there's both a main writer and a main artist of the series that have a say in things, but the spin-offs are, indeed, done by other writers and artists.
As for Emiya...I'm pretty sure I've heard the author has declared the different routes and spin-offs alternate realities in a multiverse(like superhero comics work) so, things somehow cross over at times(also, like superhero comics).