Jailbreaking requires the exploitation of a hardware or software flaw in the iOS itself. Basically the more releases Apple has, the more prone iOS is to jailbreaking because adding new content means opening up new doorways for hackers to sift through.
1) Hardware and software patching will always come, but new methods of exploitation will always arise as said above.
2) There's absolutely no point of attempting to jailbreak a beta iOS because it can easily be patched if found. Once a GM release is out, that's when it's logical to start researching for jailbreak exploits
3) Aside from exploiting the hardware or software, you need a delivery method that works with the exploit. Meaning even if you have a bootrom/userland exploit, without a delivery method that works, that exploit is useless.
