I wash my boy with washing up liquid and warm water, because I hate splashing out haha. Without elbow grease no amount of expensive shampoo's will work.
I usually get a pair of sharp hairdressing scissors and cut a chunk off where i'd like it to be, then move up to mane a little bit at a time cutting it level with whats already been cut. Takes a while, haha.
Yes! Plait up! If he has a skinny neck, do button plaits pointing upwards, if his neck is chuibbier do them to the side. If he has a long neck do big ones and if he has a short neck do lots of little ones! I was taught this, people have got smart about plaiting now to enhance their horses features xD
You can't really get rid of scars, if the tissue is damaged the hairs will grow back white. Don't worry about this really as long as you aren't doing the showing class :)
I use Lincoln glycerine spray, but i'm from the UK so I don't know if they sell that where you live :p
I'm probably no help haha, it's just what i've been taught over the years by family and instructors and stuff