While your definitions are correct, a lot of drive by downloads happen when you visit otherwise trusted pages - because the ad network servers either got successfully breached or they didn't vet their advertisers well enough (again). For example - go to cnn.com today and view the source of the page. ads.indeed.com, doubleclick.com, etc. All of these ad networks have had serious issues with serving malicious advertisements from time to time. They will allow someone's ad that uses a malware kit attacking all the Java, Flash, Adobe Reader, etc. vulnerabilities that are out there. People shouldn't get drive by downloads just because they visited what should be a trustworthy site. So yes, drive by downloads can and do come from what are supposed to be ads. They are purchased via legitimate ad networks and run on many sites.

It doesn't really matter if it is external or internal. Any time you remove it from a search index you have effectively taken the material down. If people can't find it, it doesn't exist for them. If you remove a book from the card catalog, it can exist in the stacks for years with nobody ever seeing it. Web pages are the same way. If it isn't on the first couple of pages of results on Google, Bing, Duck Duck Go, Yandex, Baidu or whatever - forget it; it no longer exists as far as the world is concerned: they will never see it. Sure you can always send out links to a few people and those people can see it. You can post links on MyTwitFace+ and a few people will see it. But for the vast majority, the index is their view into the internet. Remove it from the index and it ceases to exist for all practical purposes.