Is it effective when working to simply dip the solder wire into the flux and solder or is it significantly better to apply to the joint and why? You want to apply the right amount of flux. Too little and you won’t get good soldering results. Too much and you run the risk of the solder flowing where you don’t want it to, since solder follows flux. You want to apply the flux in the manner that will best ensure you lay down the right amount. Usually this means using a tool to apply the flux to the area being soldered and not dipping the part into the #135. SRA #135 is not runny, and if you dip the part into the flux, you might not get enough flux on it. That being said, I am sure there are people who do dip their part into the #135 flux and get good results.
Is it advisable to heat up the flux with a heat gun if it is too hard/solid? You would want to do this very carefully since you don’t want the ingredients in the flux to separate. Quick, intense heat could cause the flux to separate into disparate ingredients. While this is not too likely, it can happen.
If the rosin flux becomes dry is there anything you can do to return it to more of a paste? You would want to gently heat it until it softens up, but you need to be careful not to heat it up too quickly or at too high of a temperature.
Does dipping your soldering iron directly into the flux help keep the tip clean, are there any benefits? Not a great idea. It wouldn’t be good for the solder iron because if you get too much rosin on a solder iron, it can be hard to clean off. Also, it can cause the flux to separate. Better to put the flux on the part that you are soldering, and to then apply the solder wire and solder iron to the area where the flux is. Of course, the solder iron will usually contact the flux, but you don’t want to 'bathe' the soldering iron inside the flux by sticking it in the flux jar.