if you lists the allowed sites in your account such as bukisa.com or hubpages.com,
you still need them to add in your channels?

I think adding in the channels is only for tracking purposes. So theoretically speaking I guess you don't need to do it.

No you don't include a domainname in a channel unless you track a specific website of yours, then you make a channel-name just for this site and use the adsense code just for the named site to track it.

If you set up adsense on your sites, then google will find its way to crawl that and record your sites. You don't have to worry about it. But, always checked you google records, or get google analytics.

once you place the codes in any ads that meet adsense T&C there crawlers will find it

for allwoing you do not need to do that
i have not done that and no body do it for that purpose