Well R2-D2 and C-3PO had their memories whiped, and Obi-Wan probably knew this so when he saw R2 and 3P0 he acted like he didn't know them. Or possibly forgot about them because there were thousands even millions of droids around, so why would 2 stick out?
R2-D2 wasn't the one around Anakin has a child, that was 3P0. Also like I sais there were thousands probably even millions of Droids that were around, why would you remember one, especially after getting your legs and left arm chopped off, burned alive, left for dead. Manage to survive and then have to spend the rest of your days in a mechanical suit to live, and then get the news you killed your wife. And then say a few years later "oh shit, R2-D2, man me and him had some good times." No way, man.
R2-D2 Never needed to fly, he wasn't in any situations that required him to go to his last option.
As for the Death Star, I mean I'm sure it took some time to get it built the first time, not 20 years though, I'm gonna watch that again to make sure you're right about that. Also Empire is about 3-5 years after Episode IV.
I'll give you a question that I can't seem to figure out, how the hell is Anakin among the spirits of Obi-Wan & Yoda at then end of ROTJ, did I mis something there?