Hi.. Stretch marks don't go away since they are skin that split apart by over stretching, but after awhile looks less obvious. Tanning is something people do to reduce the appearence. Scrubs are also a good way to smooth it out. It is best to use lotion if you work out because dry skin will usually cause stretch marks keeping it moisturized is best to prevent it. Just about everyone gets stretch marks even skinny people.
About the itchy rash..Could you be allergic to some food or detergent. Also wearing 100% cotton t shirts seems to help. Nylon, polyester, or fury materials seems to irritate skin. I get itchy back too and taking a shower and using some soap helps. Also afetr shower dont wear your shirt right away give it time to air out before wearing a shirt. changin bedsheets weekly. Sweat cause bacteria which leads to itchy back so wash up and keep cool and dry and clean..