1. I am a native speaker of English. a) We never say, "the temperature is plus 12 degrees centigrade." We would say, "the temperature is 12 degrees." We might add "centigrade" if we want to be really precise, but, in normal conversation, we would not use "centigrade". Only if the temperature was below freezing would we say, "it is 12 degrees below freezing," but, if it is obvious to everyone that we are not freezing, then we say, "the temperature is 12 degrees."
b) An English speaker would not say, "it's 12 degrees above zero," unless they wished to be very precise, but, in normal conversation, an English speaker would simply say, "it's 12 degrees."
c) An English speaker would never say, "It's 12 degrees positive." The sentence would not make any sense in normal conversation. Even if the conversation were to be scientific - and very precise - a speaker would say, "it's 12 degrees above freezing."
2. This sentence is correct, except for the capital S on summer. If you are speaking of the season, a capital letter is not necessary. However, some females have the first name of Summer, so, in such a case, a capital letter would be correct if you are making preparations to welcome a female who is called Summer.