The first two questions are already answered thoroughly by the first two replies. I'm adding a little bit of information that I know about the last two question.
* Can you participate in more than one?
If you have alt accounts you can participate in different signature campaign, bounty, airdop as long as you follow the rules of the forum and don't spam. Joining the same signature campaign or bounty with alt accounts is not allowed and if you get caught you'll get tagged by the DT members.
* If you post here everyday as a habit, what's the level of extra effort involved in participating in bounties and signature campaigns?
Depends from one signature campaign to another because there are two different types of signature campaign. The first one is the most used it's called a fixed campaign to get paid you need to reach their minimum requirement this type of campaign is also being followed by bounties. The second one is pay per post signature campaigns which is straightforward you get paid for every post and there's no minimum requirement unless it's stated by the campaign manager.
If you want some examples here's a
fixed campaign thread and
pay per post campaign to see them in action.