Rusting gas lines is not hard to believe. Items in contact with water -rust - no oxygen in air there - no air period. But there is oxygen as the make up of water. Water will cause steel to rust. There can be water in the gas line as well as the tank. You can get it from the gas station if their tanks are running low and you get it from condensation from extreme temperature changes. My line along the bottom of the car was so rusted inside - it collapsed and broke in two when I removed it. You can argue that it cannot happen, but as GOD is my witness - it happened to me. My car was parked for 3 years which may have exasperated the problem. Just thinking - if rust cannot happen with gas in the tank and lines - why did they change the gas tanks to plastic from steel? Remember the "Seals-all" goop to stop gas from leaking out of rusting out gas tanks to extend their life? Sometimes truth is stranger than fiction.
By the way - I take it as an affront that you infer that DIYer's "incredibly screw up" cars. I have worked on my 560sel, 5 VW diesels, my son's 928 Porsche and 2 GM cars for several years and they run great. I have rebuilt several engines, do all my own mechanical and body work and get a lot of pleasure from it. While SOME DIYer's may not be mechanically inclined - many others are. Are you saying that contributors to this board like Mike Tangas and Hbofinger who are DIYers and have helped many others to do major work their 560 sel's are "HUMOROUS"?
Too bad you find us DIYers helping each other on this board as "humerous screw up artists". There may come a time where you have to eat your words and ask a DIYer for help with your car.