ruina is generally used in the plural when it means “ruins” of a structure or something similar. Mind you, there are other ways to say “ruins” and words meaning “remnants” or “remains” are good (and sometimes more striking) alternatives.
yes. Cicero doesn’t seem very fond of the term, but since you are talking about a church, a church that has even had time to become ruins, that doesn’t seem to be much of an issue. but if a bishop were to see the boy sleeping in the church ruins, he would no doubt use the phrase “ruinas penes dormiebat.”