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.)