Well, folks, I had tried pretty much everything to get this beautiful bird into my game. I'm running an antique setup Version 4.11.1m with SAS activator 4.0. I know, I know, dark ages stuff, but it does well on my little rig. Anywho, I got the D520 finally into the game. I'm posting this in case anyone else wants to give it a shot. I have no idea what nature of the "D520" file is that you find in the "classic" download of this mod, but there exists in Ranwers mod of this plane a file with that exact name, simply "D520".
I deleted the "D520" file which came with this mod's classic version and replaced it with Ranwers.
Voila!
Both appear fully operational now. If any modders see that I may have screwed things up by doing this, please post so that I can make a correction, but, more importantly, so that others don't repeat the error. If it IS a safe thing to do, it may help those who have previously been unable to get this beauty to fly.
And, wow, she is a doll. Thank you so much.