1. 兄はおやじに似て、くそ真面目だから、海外生活って言っても僕のとはずいぶん違ってましたけどね・・・。
My translation: "My brother takes after my father, and he's so damn serious that his life overseas was quite different to mine."
a) What is the purpose of って言っても here?
I think you can take it literally.
When saying 'livign overseas', it means something quite different from mine.
b) Does 僕の mean 僕の海外生活?
Yes.
2. やだ~まさか~。
What is やだ?
やだ=いやだ, means "no".
3. できたら40代ぐらいで会社やめて、イタリアン・レストランの経営なかんかしてみたいなぁと思っているんですけどね・・・
My translation: "If possible, I think I'd like to stop working in my 40s and try running an Italian restaurant."
How does なかんかして work? I am not sure even where the word boundaries are... Or maybe there is a typo?
イタリアン・レストランの経営+なんか + して+みたい+なぁ
4. もう一歩努力すれば、彼はうまくいっただろうに。
Translation given: "With just a little more effort, he would have succeeded."
What is the purpose of に here?
に implies that actually he did not make efforts and consequently he did not succeed.
It adds a feeling, such as regret, or thinking something a pity, shame, etc.