Question: How is Scarlet Spider still alive if he was in the crashing Hydra island?
Question: What happened to the lieutenant after Amon threw him into the pile of wood?
Answer: We do not see what happens. Avatar shows rarely have people killed on screen, so either he got away and was never seen again, or just died right there. As Amon was outed as a bender, he would not go back for his old boss.
Chosen answer: He simply survived the crash without much explanation of exactly how. However, he hid from everyone after the crash to find out his truth, so everyone just thought he died. (This is common is comics where someone is presumed dead or in a situation "no one could survive" and then later shows up alive and the how isn't explained).