Whatever reasons apple had for not using ZFS at that time wasn't some "IP shadow" as the CDDL includes a patent grant.
OpenZFS is open source and there is nothing murky or scary about it.
There is nothing stopping Linux from mainlining ZFS at the source level apart from kernel developers reluctance to give into "layering violations." Somebody can correct me if I'm wrong.
To wit: I am not sure if linking and terms of binary distribution matter after sources have merged for open source projects. If you have the source code and have right to modify, merge and distribute it, arguing about static or dynamic linking and binary distribution is like arguing about the color of your car door after the car has been made. It's inconsequential compared to amount of IP and resources put into the source code, and easily changed by any user (to a different architecture lets say).
Modifying has no meaning when it comes to binaries. But it's core to copyleft and open source. You would only care about binary licensing if you were a closed source product, and had to have ultimate control. If somebody had complete copyright over a ZFS binary, they could say how it can be used or not. The way an EULA would restrict you. Since no such copyright holder or binary exists for ZFS, and only source does, I don't think most people would stop collaboration in source code once the licenses are compatible.
Linking exceptions are for those who do own all of the copyrights and want to distribute along side open source software, and having a distinction otherwise in the open source world adds to license proliferation and makes no sense. ZFS doesn't have a single copyright holder acting on it's behalf, it has lost certain privileges because of this. I'm sure Oracle would be troll about this too, but they would probably be wrong given FreeBSD and OpenZFS.
I do think the GPLv3 fails a little bit because of the same argument. But I'm no lawyer.
I doubt (given what I heard from lawyers on the topic) that the legal system cares if a work has static or dynamic linking technology. What matters are the legal concepts which previous legal cases have been settled on.
Intent by everyone involved such as the author, the accused and the law writer. If the author intended that the work is used in one way, and the accused knew this but decided to go against it, then that carries a lot of weight. Similar, if the law writer intended the law to address a specific situation, that also carries weight.
Precedence from cases that involve derivate work. There is a fuzzy line when two works merge to create a third. Music has a large legal history, parts which are contradicting itself.
And last there is the law itself. Modifying for example is a explicit exclusive right in some places (such as the US). One case involved a person who bought a painting, cut it down into squares, and rearranged them into a mosaic version. The painter sued and won the case, arguing exclusive right to create modifications. If something is binary or source code should irrelevant to the question about if the "work" has been modified based on what the author originally created.
I think we know what Sun wanted, they wanted to make money from Opensolaris and ZFS by building a community around it and selling services and products around it. After a decade, all that's left now is the source code, and if Oracle wants to wait until ZFS becomes bigger to sue Canonical or Netgear, well, that shows their intentions as well.
As this timeline and some Googling shows: https://en.wikipedia.org/wiki/OpenZFS#History
Sun did work in good faith with Apple, and the Linux community to get them to adopt ZFS, unsuccessfully (successfully with FreeBSD). Additionally, the fact that Sun did successfully open source quite a few things (virtualbox, jenkins, openam, solaris, staroffice, netbeans, etc..) and relicense Java from SCSL to GPL, makes their intentions towards the open source community pretty clear. Yes, they wanted to make money, but they probably open sourced and created more open source communities than any other company in SV history.
Now, about modification, any open source license listed by the FSF will grant modification rights to users. I don't think compiling is making a derivative work. It's like unfolding a chair to sit on it. It's just a part of normal usage of software, you can decompile a binary and learn from it, also normal usage. The compiler is a tool, like a screw driver or paint gun that will let you assemble a chair or paint your car. Reading and learning from source code is usage too. Modifying the actual source code would be a real modification and could be making ZFS work on an Raspberry Pi, which is allowed by open source. Given that Sun wanted ZFS to be widely adopted in open source, they adopted the CDDL to let people modify ZFS so it could be used by OSes other than Solaris. This is what the OpenZFS community enables, and is completely compatible with GNU/Linux or Apache open source norms. Oracle might come knocking for money, but that's not the history of Sun or current ZFS contributors, who are just out to make better software using the open source process. They would probably not disagree with what Netgear or Canonical did, and if they did, it would be on the OpenZFS mailing list and in a news story or two. It's not.
You can't copy books and sell them, and I can understand you can't modify an original artwork and not affect the copyright owner's rights. You can correct an error in a book or claim inspiration from a painting to make another. You can't claim copyright if someone uses a binary in a VM when you didn't intend it. You can give others the right to modify source code, and ask that others do the same. That is open source and the GPL. OpenZFS, FreeBSD, have as much standing as Oracle, which is really none, to actually stop someone from porting ZFS to anything they would like and distribute it along side proprietary or open source software.
I have heard a few different version of the intention behind ZFS license, through I can't say that for a legal case I have enough information to have a definitive opinion. Some people say that ZFS license was created with the explicit purpose to be incompatible with linux in order to not compete with solaris, but I would agree that Oracle has a thought case in court if they want to wait only to sue later. That is a practice that the legal systems tend to strongly dislike.
The other side is of course each one of the linux developers, each holding the full power of copyright. To cite SFLC, no free software developers have ever sued an other free software developer over license incompatibility, so its very unlikely to happen with ZFS. Such court cases really on happen between companies.
So to sum up, a case over ZFS is very unlikely, but I would not bet on what would happen if android suddenly started to use ZFS.
I think the Sun leadership at the time wanted ZFS to be on Linux up to the point of licensing it under the GPL. The employees wanted a more BSD-like license[1], so that's the correct context under which you can look at how Solaris was licenced. It's not about being in Linux or not, it's do we want Solaris to be under a more BSD like or GPL license. I think this conversation was bigger a decade or more ago, and frankly the GPL has had more commercial success since then. If Solaris had been GPL'd is an interesting thought experiment, and too bad it's just that. Netbeans was GPL and CDDL dual licensed.
I wonder what Linux Torvalds thinks about merging ZFS into Linux now, he wasn't too keen a decade ago. Sun is no longer around, someone worse like Oracle has taken their place. A couple of lessons for the open source community here, I think. And Brian Cantrill nails in on the head in the youtube video link.
ZFS will need to be on Linux first before it can show up on Android or media centers or gaming consoles, and I don't doubt Oracle's ability to find a way to patent troll anything. But it will be just that: patent or copyright trolling.
To add a couple more things: Oracle has been developing ZFS sans open source for the past few years as well, which means they've stopping caring about OpenZFS and/or have lost the right to.
Canonical, Debian and SFLC have really done with right thing by distributing ZFS on Linux, using AFS as a precedence. I hope more merging like this can happen in open source in the future.
