1. yes if you add too much fragrance it can end up pooling on the top of your finished dry candle. Go to the website (or look in the paperwork or even in some cases on the bottle) of the fragrance oil company to see how many ounces they recommend per ounces of wax. It will depend on the kind of wax and oil. If essential oil you use less (e.g. half an ounce per pound/16 ounces of wax - though paraffin wax doesn't take so well to most essential oils unless combined with a fragrance oil. Soy and beeswax I think take essential oils better.
2. If you put in your fragrance too early before pouring into your mold/container, your wax will still be very hot and the scent is at risk of burning off so your candle will be weakly scented. You're supposed to heat up your wax initially hot enough to guarantee it's fully melted, but then you're supposed to let the wax cool on the stove/in the pot just enough to allow for a smoother candle that holds the scent (and possibly also color) better without burning off. If you pour at too high a temp, the candle may react more extremely to the sudden temp change (one reason some say to heat molds or containers before pouring). So you might get the frosted look or cracking or bubbles, etc. With soy wax going into containers for example most directions say to pour at a cool enough temp (e.g. 150 or maybe less) and to pour slowly so as to avoid bubbles.
3. Flashpoint is the temperature above which that particular wax or fragrance oil would be at risk of catching on fire. Something to avoid, me thinks.
4. I'm not as experienced with container candles but my understanding is that in some cases it can be the same case as with pillar candles, meaning you might want to add an additive if you have a straight 100% type of wax that would benefit from having an additive mixed in. I think some kinds don't require it though. I used a straight soy wax the other night and the directions that came with the wax never mentioned anything about additives being helpful. Some additives (google) make candles look more opaque and supposedly hold color and scent better (vybar or stearin?), and other additives can deliberately give the candle more of a frosted, mottled, rustic look (e.g. mottling oil or "snowflake oil." Although I've read some if not most fragrance oils contain something that does the same thing, so mottling oil isn't necessary in all cases or less of it is needed if using a scent. But really, if memory serves, pouring into a cold metal mold is what really helps give the frosted rustic look - or putting the mold into the fridge or freezer for a bit while it's drying. Another type of additive would be something like a dried herb or flower, like lavender. Some people sprinkle this on the top of their candles. Not sure if I would do that cause it seems it would eventually burn and look bad but I could be wrong.
