I'll try... Apologies if I get anything wrong, as I'm new here.
1. Finishing feed is basically any feed used to plump chickens up before slaughter. Chicks raised on medicated feed then switch to finisher so the medications have time to get out of their system before being turned into meat. Usually finisher is slightly lower in protein than starter, but higher protein than layer feed.
2. Age to kill is very varied. Commercial hybrids get oversized for their legs unless feed is restricted, and this can happen at any time from 5 weeks of age, though more commonly 10 weeks. For this reason commercial meat birds are usually killed at 5-6 weeks. Dual purpose purebreds often take up to 24 weeks to really flesh up. Deliberate crosses e.g. cornish x Rhode Island Reds are usually good eating at around 16 weeks.
But for all birds (unless you skin them) it's best to avoid killing during times when pin feathers (early feather buds) are growing, which occurs during moult. Chicks moult firstly between one and six days of age; then they have partial moults at 7-9 weeks, 12-16 weeks, and 20-22 weeks, depending on the maturation rate of each bird. The best time to kill is outside these moult times, when they can be fully plucked without too much extra work removing pin feathers. If you're skinning it doesn't matter when you do it. I do it just before the birds crow and start trying to tread hens, otherwise they very quickly become tough (16 weeks).
3. Resting the bird is to remove rigor mortis (when the muscles all go rigid). Rigor mortis sets in within 20 minutes of death, so if processing takes longer than 20 minutes then the bird will be really tough. Resting the bird for at least 24 hours in the fridge (and up to 48 hours is best) will remove rigor mortis and make for the most tender carcass.
4. You don't have to scald/pluck; I always dry pluck (if not skinning, which is far, far easier), as there's no horrible wet chicken smell, and no danger of cooking the skin if you dry pluck. But there are 4 factors related to ease of plucking.
One: relaxation at death. A frightened tense bird will have tense muscles.
Two: method of dispatch. Head chopping always sets feathers hard; pithing (debraining) softens feathers if done correctly.
Three: if rigor mortis sets in before plucking finishes, feathers will be set tighter as well. For that reason you need to start dry plucking *immediately*.
Four: age. Older birds have harder-set feathers and also tougher skin (when it comes to skinning). The only reason not to skin is if you want to roast. I prefer poached chicken anyway, and I find fried chicken with the skin on a little too fatty for my taste. (Not that I've got anything against fat.)
Hope this helps!