Well, like the imabi link in the post you referenced indicates, ~てたまる “[shows] a strong resolution of not having something be in such a state/condition.” Here, that state or condition is 封じられる. I think the passive is probably what’s meant (over against the potential, which has the same conjugation for 一段動詞), so “being prevented, being forbidden,” that kind of idea. Putting these together, I guess 封じられてたまるか would be something like, “I would never be prevented,” maybe?
The following lines in the song seems to match with that.
君に届くまで終われない。苦しい時聴けばいい存在になれるまで。
It cannot end until I reach you. Heed these painful times until I can be (there? with you?).
聴く is hard to translate here because I’m not sure of the nuance that particular kanji spelling has over 聞く, but since ばいい expresses emphatic advice, I chose “heed.” 存在になれる is kind of hard too. The dictionary meaning of 存在 is simply, “existence, being,” but I’ve seen it used more along the lines of one’s presence or one’s existence in a personal sense (e.g. 先輩あたしの存在に気づいた!). Presumbly, the vocalist is talking about being present with 君 (you–whomever the song is being “sung to”).
Take that all with a grain of salt. I’m a fumbling language learner trying to apply what little I know. Hopefully, that’s at least helpful and not completely wrong…
Oh, one parting note. From having previously used song translation (of my favorite Japanese band, Tears of Tragedy) as a learning tool, I can tell you that it’s often especially hard to do. Song lyrics tend to be poetic and literary, which means you’ll find more rarely used words, expressions, and senses of words you thought you knew. If you’re sticking with pop music, then maybe to a lesser degree, but in general, lyrics are much harder to translate than ordinary prose.