Making sets
 

You would most likely hear something like:

毎週3回、1回2時間日本語を勉強する。(no particle in front of 日本語)

6カン入りの玉子を4人前下さい。

I left out the word 'sushi' because this is something you would only say at a sushi restaurant. Your chef or waitress knows you're talking about sushi and not anything else that's made with eggs. カン is the counter for sushi and 6カン is pronounced 'rokkan'.

No, you would sound like an small kid if you counted sushi pieces using 個. I live in Tokyo and I go to a sushi bar almost once a week. Besides that, I'm a native speaker to begin with!! Counters in Japanese are never very simple.

~入り　means containing ~ (~ is a number). You will sound much clearer if you include ~入り because you're putting more than one counters in a very short sentence. By the way, that word is read いり and not　はいり if you were wondering.

Yes, as far as I know, suhi is it in the daily lives of many Japanese people. That's why we often don't bother to write it in kanji, which is 貫.

The counter for volumes in books is 巻, which is also read かん. But with this one, we always write in kanji...unkess a kid hasn't learned it.