ORIGINAL: guver
I'm no expert. I will take a stab at ? 1 and 2 as well.
If a lot of weight is needed applying it out at the tip is going to mean a lot more per gram. It probably doesn't matter , but I also agree with jw to remove weight rather than add it. I have taken the approach that IF I had to write on my prop I would add weight with a marker , but if not then I will sand weight off. I imagine it is so insignificant that it doesn't matter. Also my ? isn't really answered either , but jw is probably right about that as well. It may not matter. I was concerned about roughing up the biting face of the prop.
sorry I can't even guess about the other ? you have. Again, I'm no expert, but the few props I've worked with have me convinced that getting the prop on straight without runout is as important if not more important than the actual balance. Some of mine aren't too good though. The cheap models I have are poor quality.
I used to remove material, but I read in a number of places that it's better to add material because removing material can affect the structural integrity of the blade. Whether I'd be removing enough material to make that true, I don't know, but I figured better safe than sorry. As far as efficiency goes, I'm not sure if that's exactly what I'm trying to get out of a balanced prop. I usually do it as a preventative measure to keep the prop from vibrating the airframe or destroying motor bearings. I've never had any of those things happen, but then again, I've always balanced my props in one way or another.