How long can uncooked brisket be refrigerated after defrosting

The water pan does affect humidity in the cooking chamber and therefore, it does change the how quickly moisture evaporates out of the meat. The question really becomes - "Will the meat be moist enough without the water pan?" Which, to a degree, is personal preference. Some choose to not use a water pan and baste instead, some use both and some neither.

I would recommend trial and error. Brisket is relatively pricey so I would recommend using pork shoulders. Smoke one with the pan and one without and see if you think the moisture content is satisfactory. The briskets won't cook exactly the same but the relative moisture level in the shoulders should give you a decent estimate.

I personally always use a water pan, but I don't usually fear meat from a smoker being "too moist", and it does wonders for temperature stabilization. I also have a larger smoking chamber than a WSM so it's possible that too moist is a larger concern for you.

One last note, while cooking too hot for too long will inevitably dry out your meat, cutting a brisket or shoulder without letting it set for at least an hour (ideally 2) after cooking will also dry it out.

Pretty much no, they are totally non-alike. The beauty of brisket is that it is full of connective tissue and other odd stuff that translates to succulence when cooked low and slow. Round is pretty much the opposite, it has very little in the way of interest.