なく after a verb, what are the uses?

Question

夕暮れで 二人 少しずつ見えなくなっていって In this case I suppose it 見えなく(invisible) + 成っていって（becoming）.

But in this case below how would it work?

サヨナラ 会えなくなったって 僕らは続きがあるから

Answer 1

見えなくなっていって

見える dictionary form of verb -- 'to be visible'.

見えない negation of dictionary form -- 'to be not visible'.

見えなく continuative form of 見えない. This form is needed when preceeding なる (to become).

見えなくなる 'to become not visible'. Another example -- 暑い + なる --> 暑くなる = become hot.

なって te-form of なる -- 'become ...'.

なっていって -- なる + te-form of いく -- 'start becoming ...'. See this link.

見えなくなっていって 'start to become not visible' = 'start to disappear ...'

my question is exactly what なく　means.

In summary, it's the continuative form of ない which is needed when you want to say that something becomes something. For an i-adjective or ない replace the final い with く and then add なる.

会えなくなったって

会う dictionary form of verb -- 'to meet'.

会える potential form -- 'to be able to meet'.

会えない negation of potential form -- 'to be unable to meet'.

会えなくなる just like above -- 'to become unable to meet'.

会えなくなったって ' even if we'll become unable to meet. This 'even if' form is made by converting the verb into past tense and adding って i.e. なる --> なった --> なったって.　See this link.

Answer 2

In the first example, your understanding is correct. In The second example, って　has a different usage than the first. It means "although"

In This webpage, at the very end, section 4 part 2, it gives a usage of って that is the same as your second example.

（た、だ、からなどに付いて）譲歩を表す。ても、でも。たところで、だとしても

(attached after ta, da, kara etc.) To show compromise. means temo, demo, ta dokoro de, da toshidemo