こんにちは！ ２については、 ・残業しても　⇒　残業（名詞）＋し（「する」のテ形）＋ても（仮定の逆接） ・残業してでも　⇒　残業（名詞）＋し（「する」のテ形）＋て（順接の接続助詞「て」）＋でも（仮定の逆接） 「ても」and「でも」both work as assumption/contradictory conjunction, but 「てでも」has 「て」, which works as resultative conjunction. In that sense we have positive sen\\tense after 「てでも」, but we have negative sentense after 「ても」 Usually we don`t do overtime(but we always do in Japan lol), but we have to overtime because we have to finish the task; This is resultative conjunction.(てでも) Iin opposite, if we put only 「ても」,the first sentense "we do overtime" and second sentense "we have to finish the task" is connected by contradictory conjunction, and it sounds strange for we Japanese. I`m sorry my answer is getting more and more complex...if you have further question, please let me know.