I worry that my question after edited won't be seen. Should I flag it? And in case it's still not qualified to be reopened, can I ask on the flag to tell me which one are still unclear?
No, you shouldn't flag your closed questions so that it will get reopened.
Flags are not for that purpose. Flags are designed to bring inappropriate content to the attention of the community.
From the help center
Flagging is a way of bringing inappropriate content to the attention of the community.
If you edit your post, it will be immediately bumped to the homepage. So, users will see that post. More over all the closed questions will be put in the reopen review queue if they are edited or they have a reopen vote on them. So, the users with close/review privileges will attend the review and reopen if the standards are met.
So, there is no need to flag your own post or any other post. They will be declined.
Can I ask on the flag to tell me which one are still unclear?
You can ask it in the comment section. If the post is flagged and reason is asked there, only the moderators can see it but in the comment section, all the users can see it and respond you what is wrong with the post. It helps in reopening the post quickly than the moderator intervention flag.
But I saw some advice on other meta posts that I should flag to get further attention for reopening!
This is severely outdated advice, from the time when recent edits wouldn't push questions into the reopen queue, and the only way to draw further reopen votes beyond the homepage bump was to flag or request on meta. The system has changed considerably since then, and the "moderator intervention" flag is now to be used only in cases where only a moderator can handle things.
About requesting on meta: per the most recent FAQ, you should only do so if 1. you have exhausted all appeals (editing, voting, and commenting) but it still wasn't reopened, or 2. you commented but didn't receive that much clarification about why your question was closed.