>What about when companies spin-off subdivisions as separate companies? Can they "legitimately" own a patent and then spin-off a troll with that patent as its only asset?

Spin offs aren't mergers. If you create a new entity as a spin off and transfer patents to it, it wouldn't be the original assignee anymore, so it would have to practice the invention. A spin off that doesn't practice the invention is a patent troll.

>Or what if the original inventor own only part of the company?

It doesn't matter who owns the company, it matters who owns the patent. If it's owned entirely by either the inventors listed on the patent, or the first company they assigned the patent to immediately on issuance (typically their employer), then they wouldn't need to practice the invention. If it's anyone else then they would need to practice the invention. A company's single exclusive designated successor stands in for the original company if the original company ceases to exist. You can't split up the patents in any way or else all but the designated one of the new owner(s) will have to practice the invention.

> Anytime you give property rights to someone but then try to restrict them, there will always be away to wriggle around the restrictions when sufficient money is at stake.

Any time there is money on the table someone will try to take it. That's a poor argument for not doing anything about it.

>Eliminating software patents might indeed be a good, crude decisions which is better than the status quo, but the abstract nature of software does not in principle prevent novel, patent-worthy ideas from being reliably identified.

Software itself is an abstraction. The only types of patents that software could infringe are patents on mathematics itself (e.g. compression or crypto patents) or patents on abstract ideas that can be built on top of mathematical primitives, because that is all software is. It's abstractions built on abstractions built on abstractions built on mathematics. And none of that is even supposed to be patentable. Contrary to popular belief, patents aren't supposed to be granted for ideas, they're granted for inventions. An idea isn't an invention, the invention is the thing you make after you have an idea. You patent a nuclear reactor design, not E=mc^2.

The "problem" with software is that there are arbitrarily many practical inventions that implement a given idea. Patenting any partial subset of the possible implementations would be pointless, because any competent programmer could come up with a hundred other different ones that implement the same idea. But patenting the idea itself goes too far. Which is what they've been allowing and what causes all of these problems. It's why in software accidental infringement is the rule rather than the exception and as a consequence everything infringes everything.

People have intuitively understood this forever. It's for exactly this reason that literary works aren't patentable, because it would cause the exact same thing. But software makes peoples' brains explode because we created virtual worlds where you can go simulate human interaction and even buy and sell fake stuff for real money, and at some point we went from calling those interactive works of fiction names like World of Warcraft and Second Life and to calling them names like YouTube and Facebook and the App Store.

At some point we lost track of the line that distinguishes a patentable device that physically holds your CD collection from an unpatentable work of interactive fiction that allows you to see a picture of such a device on your screen and pretend to interact with it, and we started issuing patents for simulated "inventions" that exist solely as abstract ideas in software literary works. But it was never supposed to be this way because of exactly the mess that allowing that has caused.