Backlinking / article marketing question

Your understanding is mistaken.

The theory that you need to change a proportion of the content has grown up and become widespread because many people mistakenly believe (a) that having multiple copies of same article in different places is "duplicate content" (it isn't - it's syndicated content), and (b) that as a result it will somehow incur some sort of SEO "penalty" against your own site which is linked to in the articles and/or in their resource-boxes (this one's just too ludicrous for me to "go there").

Most of the time, most of those subsequent identical copies of your articles will be indexed only in Google's "supplemental index" rather than in their main index, but their backlink-value won't suffer at all from that.

Personally, I sometimes change those articles a little bit (just the titles, keywords and resource-boxes) but that's for SEO reasons of my own and nothing to do with any fear of "duplicate content", "penalties" or any other nonsense. They're still "backlink only" articles by the time I've done that: nobody (to all intents and purposes) is actually likely to read them.

Yes, you can - but you don't need to modify them.

None. Unless you want them all to be indexed in Google's main index rather than the supplemental one. But if you want that, you need to change a lot (maybe 70% for each copy?), and make sure it's still a high quality article which people will read and will attract traffic just like the EZA copy does, and so on. But that necessitates making your article fit for human consumption rather than just search-engine-spider consumption ... and that's a whole different proposition. For myself, I'd rather write a new article than do that (it won't take any longer). There's no real point in changing it "slightly" apart from what I've mentioned above.

There are hundreds. It's a huge industry. Whether they're of any real benefit/value is for each of us to decide for ourselves.