Having a patent doesn't grant you the right to use it (for example if someone else owns a patent for a similar technology you still have to make sure you aren't infringing that one). Defensive patenting only means that you have your own patents to assert against someone else who might potentially sue you.

I know very little about the topic, but is that a thing? Surely just publishing (without asserting ownership) could establish prior art?

Any sufficiently large organization tends to become host to a population of defensive lawyers, which then produces patents as a side effect of building themselves a nest out of legal documents. Isn't nature beautiful?

In theory, yes. In practice, patent examiners mostly only look at patents when checking for prior art. Once someone else gets a patent for your invention you'll face a costly legal process to invalidate the bogus patent.

While that may be true for generically publishing the work online, there are dedicated disclosure journals that are included in prior art searches by patent examiners. If you want to publish as an alternative to patenting, this is where you do it.

"My reckons" are not generally reliable when it comes to law.