Technical Preview is what Windows 10 preview is (was) called. Preview for Developers on the other hand is the preview version of Windows Phone 8.1, which is deprecated so to say. It required a developer account - a limited one, obtainable for free through the app studio web application - to participate. The Windows 10 Tech Preview requires a Windows Insider account, which you can get for free too: https://insider.windows.com/
Windows 10 for devices < 8" (Windows phones and small tablets) will follow the same path. Beware though, all these builds, regardless if they're for desktop or mobile devices, are beta builds. Far from being final. And they're often buggy and unstable. So if you participate, you do that on your own behalf.
Yes. The first builds of the TP for phones will most probably be buggy and crash quite a bit or have functionality that simply isn't finished.
This won't stop millions of people installing the very first build on their primary phone and promptly complaining that something "vital" doesn't work.
Well I must say that I am torn between getting the earliest preview or waiting for the 2nd or 3rd update. Maybe I will wait to see comments of how it works, if the known bugs seem important to me. The problem is that I only have a Lumia 920 and I really love it. (I also have L800, but with a shattered screen but it no much to the functionality of wp8.1.1 )