1) '절대' and '절대로' ― their meanings are identical, nuances being scarcely different. 절대 그곳에 가면 안 돼. 절대로 그곳에 가면 안 돼. (You must not go there : same) 2) Yes, you must use '절대' or '절대로' in only negative sentences. When positive, you use '무조건', '반드시', '꼭' and so on, instead. 절대로 가지 마! (Never go there!) *절대 가! / *절대 태보해! (Must go???) (Must do Taebo???) They're all wrong expressions. ※ 絶対できるよ。(It must work. / You can do it.) (In Japanese it seems you can use 「絶対(zettai)」 at positive position, which is impossible in Korean grammar. Better not confuse it.)

Also.. is 절대 used in negative sentences?

I very much doubt that. ~로 is a suffix, meaning something like "by/of/in". Like travel by train - 기차로 간다, speaking in Korean - 한국어로 말한다, it's made of wood - 나무로 만들었다.