1) I know him very well. I recognize him by his footsteps (the sound).
2) She doesn't have A/THE chance to tell us her opinion.
Can we rewrite it like this: Sie hat keine Gelegenheit, ihre Meinung uns zu sagen. = Not totally wrong, but very weird sound. No native speaker would place it like that.
Ich habe KEINE Zeit, dir meineN Brief zu schreiben.
3) complicated, but I'll give it a try:
gemeinsam: mostly used for activities as a community
zusammen: goes best with "together"
[Sorry, may not be the best explanation]
4) aber = but
sondern = ... but rather / ... but also
5) no difference