What is the highest threshold a string can be safely tuned? I've read that it's 1/2 step above standard; however, the B string is tuned up to C# in open A tuning. So what gives?

You can't make a rule of thumb like this very meaningful. You have to look at the whole picture. Worry more about the guitar and less about breaking a string.
If you tuned all 6 up a 1/2 step, you could easily warp the neck. Most strings would probably stand the extra tension, but some might break.
If you tuned some up and some down, the total tension might not go over the original tension on the neck (but it might still warp it if it's extremely unbalanced off to one side).
If you change the tension on the neck, it will change shape. To get it straight with the new tension, it may need a truss rod adjustment. When you go back to the old tension, it may have to be adjusted back again.

If you exceed the maximum rating (varies from guitar to guitar), you will warp or break the neck.

For acoustic guitar and resonators I would never recommend you tune above standard. Tune down around the highest raise instead and play in a lower key and use a capo or else use lighter strings.
For electric (lighter strings on mine) I would put a couple strings higher than normal, but I would keep my eye on the neck straightness very carefully.
For lap steel I tune within a range of +/- three half steps around the rated note, because a lap steel won't matter if it warps a little. You are more likely to break a tuner gear than warp a lap steel.


There is a formula that calculates pounds of tension from string material, diameter, scale length, and tuning note.
The formula is on this web page:
http://www.liutaiomottola.com/formulae/tension.htm

There is a good website illustrating this here:
http://www.pacificsites.net/~dog/StringTensionApplet.html

Here is another:
http://www.mcdonaldstrings.com/stringxxiii.html

This question is complicated by the fact that some guitar necks can handle it and some can't. Also the age of the string plays a part in whether it will instantly snap when raised up.
In general, if you are going to use strings higher than their rated tension, you will have more stress on the neck and more string breakage. If you overdo it, something will give, either neck or string. If you just push a little beyond the edge, everything may hold OK. You can use the formulas and calculators to tell you if you are doing something safe or dangerous for the strings and the guitar.