It only works if you have tons and tons of time to do research after nearly every sentence. It would be more optimal if we had a short lesson in the notes (notes are terribly lacking for this lesson!) following exercises that allow practice for that lesson. Then introduce a bit more, and so on. Duo prefers to dump a massive amount of vocab and many new concepts at you with no explanation in the hopes that one day eventually you will figure it how. Its one approach and I suppose for some it works, but it hardly seems optimal
http://italian.about.com/od/grammar/fl/Indirect-Object-Pronouns-in-Italian.htm has information in indirect object pronouns. Definitely gli.
The verb "mancare" woks like the verb "piacere". FAQ #10 http://duolingo.com/#/comment/233855
The subject is what is missing.
"Manca una forchetta" = "A fork is missing"
"He is missing a fork" = "A lui manca una forchetta"
"Manco una forchetta" doesn't make much sense. "I am missing a fork" would be "Mi manca una forchetta"
Have a lingot - this is a great idea! I get that this whole program is based on us learning as we go by mistakes, and I have no problem with that. It's not a competition so I don't get frustrated if I learn something by getting it wrong (the best way to learn). I feel like I'm cheating if I use lingots to erase an error (which is why I have 70 of them), but to use them for extra functionality would feel better.