1. Can "Remote Preview" be used on an offline LAN network?
Much like the multiplayer feature, remote preview uses a signalling server to broker a direct peer-to-peer connection between the devices. This means you do have to be online for the devices to find each other, but then the data can be transferred over the LAN if the devices can reach each other that way. Related is:
2. When using Remote Preview, how long do the builds stay on the Scirra servers?
Your project is never sent to the signalling server.
When you use the mobile app build server, it does send your project to the server so it can build an app from it. Then it immediately deletes the project. It does not hold on to it long-term.
2. How do I use the equivilent of "Fullscreen: Crop" in C3, as this option no longer exists in C3?
It was removed because it was difficult to support, didn't seem to have much use, and the other fullscreen options all seem like much better options since they handle aspect ratio and scaling for you. You can probably do what you want with some combination of the other options.
3. More mild trivial quesiton, but is there a plugin to get dates/times from the users device?
There's not yet an official feature, but I believe there are third-party addons covering this.
4. Privacy. I had lightly asked about this in a blog post somewhat recently, and I have read the Privacy policy, but I was curious specifically what degree of information is tracked by Scirra?
Our privacy policy is our official statement on the matter, but the editor only collects anonymous information and we never share the information with anyone else. It exists solely for us to maintain the quality of the product, such as by observing crash reports and fixing problems that we can spot that way (such as in this case). We do not collect or track any of the content of your project, such as anything you type in to the editor, and we have no interest in doing so. Also if you use any tracking blocking features that block Google Analytics, you will be completely opted out of this.
A. Does the Multiplayer plugin work between C2 and C3? (Could a c2 build of a game communicate with a build in C3?).
Yes, it should do (I haven't specifically tested this but the code is compatible).
B. Can you add multiple "Multiplayer" plugins in a project? Or, if not, can you connect to multiple servers on the one instance?
C3's multiplayer plugin is basically the same as in C2, and that's not supported.
C. Can the official plugins be downloaded and modified?
No, this caused appalling compatibility nightmares, it was often unnecessary for the intended feature in the first place, and we ended up dealing with the support burden.