Yes if you have a site that has traffic these can be great streams of revenue. If you google my name and check or Adsense check you will see me holding a check and what can be possible :)
Yes you can do it with a lot of knowledge and work !
Yes, if your site draws a high volume of traffic in a good niche. We have 4 million visits a year (over 6 million page views) to our website, which is law related - a good niche. AdSense generates substantial revenue from that, more than what most people would consider a full-time income. We also get revenue from other sources. We tried affiliate marketing but it didn't work for us. Might work better in other niches.
What some attempt to do is make a living off of just one website. That's very very hard. You must have millions of hits and that is a daunting task to get to those numbers. One thing to consider is having a couple hundred niche domains. Content drives people to your sites. That can be self generated or people putting content on your websites (even better). Imagine making say $200 per year off of each website. Is that realistic? In some cases yes and in some cases no. I own over 1000 domains and have been doing this for around ten years. Some domains generate $50 a year and others generate $500 in a month. I know every business has it's own numbers, but what I am seeing throughout all my sites is it take around 1000 hits to generate about $3 to $9. So do the math. If you have 10,000 hits the high would be $90. Again, that varies based on the domain and how much links are paying, but those are averages. It takes a lot of work. It's not easy unless you have the next Facebook, PlentyofFish.com or myspace. Possible yes, but not a get rich quick business. 1000 domains cost around $12k to $18k for renewals, hosting and different software for the domains. That's per year! If you build a pretty good site you have a much better chance of selling the website for more money than you can make in revenue.
Many people in my industry have attempted a min-site model to monetize their unused domain names. My impression, which is based on a large amount of admittedly second-hand information, is that this business model is seldom viable. Several years back, the story might have been different.
If you're going to try monetization with AdSense, only unique content will do. Google is notorious for suddenly canceling AdSense accounts that flag their system for any reason; and there is seldom any redress, even when such cancelations are unwarranted. So don't invest your time in this monetization strategy unless your content is unique and all your SEO and traffic 100% white-hat.
Most people seem to pursue a mixed monetization model. For one thing, those income streams add up. But I'd stress that it's important not to put all your eggs in any one basket. In particular, I distrust the Google basket. If your livelihood depends on upstream advertisers, then diversify.
Affiliate marketing definitely works, depending on the niche, the site, and the affiliate program. But no 2 cases are alike.
Lead generation can be very profitable as well, if you know how to do it.
Yes, you definitely can. And if you're extremely good at it, you can bring in executive-level pay as well, depending on the niche and verticals you are in (and your ability to negotiate better rev share with networks). Even though things are getting more and more competitive now that Google has tightened the noose industry-wide, the onset of mobile and apps has made things a whole new world of opportunities.
At my current job, we deal with over 250M unique user a month through millions of domains/zero-click redirects, many of which lead to adsense or lead-gen type landing pages. May publishers monetize this through the ways you are referring to. And anecdotal insights from my friends (and people at conferences like Affiliate Summit) will just show you that people are indeed making good money from this type of marketing.
Just like any "passive" source of income, building AdSense websites takes a lot of upfront hard work. Just take a look at websites like Huffington Post, and Mashable. Arianna Huffington and Pete Cashmore are making a lot of money these days and everyone's so excited about their stuff not remembering the hard work that went into building the websites in the first place.
Yes you can if you generate enough traffic with your website or blog. I would also highly encourage you to use website popups to collect email addresses of your website visitors so you can build a large email database that you can blast "exclusive deals" from your affiliate advertisers.