They do the same job. Which one I use is determined by my access to the fittings and pipe.
Yes. There is an applicator bush.
Yes. It works very well.
yes, it is top quality and the preferred way for threaded pipe. (do not put on any flared connector).
There can be multiply reason why a leak occurred from thread quality to not tightening enough initially. A thread tape can be used in conjunction with the sealant.
There is a NG rated tape that you can use. However, this is better for ensuring a seal. I have used tape (ng rated) and pipe dope. Tape is cleaner, but this will give you a leak proof seal every time. It will not blow out.
Works great with metal fittings. Not so sure with plastic. I use it for that application. Easier to apply.
You can use the Pro Dope for this application but we would not be able to give a recommendation on your angle valve question. We would recommend reaching out to the manufacture of the valve for that information.
Yes. It should wok well.
Sure it will. It works really well.