Both are acceptable and they express the same meaning.

If you are saying “I had two meetings today” then you can say “我今天开了两个会（议）。” If you are saying “I’m going to having two meetings to attend today” , you need to say “我今天要开两个会（议）。”

为了买面包，我排了一个小时的队。

粤语is more standard, but since many mandarin speakers aren’t really familiar with Cantonese and they think Cantonese is the language comes from Guangdong province, many of them will use the term “广东话” as well.

Usually we will say “something 有助于……”，so if you add a subject front, the sentence is correct.

In Chinese, generally we have two ways to express “re-“. The first one, if you have done something wrong, and you need to do it again from beginning? you can use “重”，”重做” redo，“重写”rewrite. The second one, if you wanna say someone repeat an action or an activity he/she has done before, you can use “再/重”，”再想想” rethink/reconsider. But the term of “repeat” is bit complicated in Mandarin and we have different expression depending on different situations.

就 and 再 have very different meaning. 就 indicates an action doing fast, period of time is short or one thing happens easily. “你想想再买吧” means the speaker suggests someone to reconsider his/her decision of buying something. So we cannot use 就 in this situation.

我觉得你理解得很好！