About Anderson taking some parts from previous scripts for his; It could be true. Maybe he didn't do it on his own decision but because somebody else liked those parts and asked to keep/add them in some way in the script they would went with eventually, it wouldn't be first time that some producers of whoever pick bits and pieces of what they like in early scripts and keep in the final product. Like for example, any of you seen I Am Legend (2007)? That part in the middle where Neville gets trapped and then he and his dog fight with monster dogs was pretty much only thing which was left unchanged in almost each version of the script all the way back from 1996 when first script was written. And in between it went through few different writers and over dozens of different scripts.
If i'm not mistaken, McElroy has "Story by" credit in 2002 RE film (but not anywhere else). It can get complicated when deciding how much credit writers deserve and do they need to have their names attached, but to have a Story By credit on the film could mean that there's lot more of the work from original writer(s) in the final film than what may seems.
Anderson's Undead script; Like you guys, i read and heard various info about this one. Wish i could help you with more info, but there's not much i can add which is not already known, i did heard how it was pretty much like the 2002 film but with some changes (obviously), but it's not like that's something which anyone couldn't figure out themselves. Supposedly original script and each RE draft Anderson did had different endings, which could explain why only one of his drafts we do have has very different ending than the film (and parts of which seems like he re-used for a scene in fifth film), and why film had two endings filmed.
One thing i do think is interesting is how Anderson's earlier drafts only had zombies and Licker, since that rumored "Evil Dead meets RE" script Bran was talking about was said to only have zombies and Tyrant. Makes me wonder did Undead also had some non-zombie monster in finale.
And here's that article i read where possible reason for McElroy's script not being used was mentioned;
Ok here is more clarafication on the alleged 'Fourth Living Dead' flick and George Romero's involvement. This comes from an anonymous (to yall) agent that scurries about in an old neighborhood in Pittsburgh, hungering for human flesh.. The...
www.aintitcool.com
July 29, 1998
"George is currently at work on the new screenplay for RESIDENT EVIL, which he has contractually agreed to make through its licensee, Constantin Films (a German production company which released DAWN OF THE DEAD in that country). The original screenplay, by SPAWN writer Alan McElroy, has been canned due to terrible story structure."