“It appears, emphasis appears, that when someone goes to the front of the church and starts singing that he sings alone until he raises his hands up in the air and everyone sings along with him? Have I interpreted this correctly? He/she sings by himself and the congregation joins this person for what I guess are refrains?”
Right, generally the cantor/priest will raise one hand when it’s time for the congregation to sing the response.
“Should I avoid certain churches when visiting? I have upon a couple of occasions found myself in a really intimidating place. Like today in St Paul’s Cathedral in Manhatten. It was a case of “Wow, Lord, nice place you got here.” and I wasn’t sure that I belonged there…”
No way! You are welcome to visit ANY CC.
"What about Holy Water? "
You can use it to bless yourself

. You touch your fingers to it, and make the sign of the cross(up-down-left-right).
You can participate in all aspects of the mass you are comfortable with except for Holy Communion.
As far as some of the songs during mass, Those are the same with different tunes, but I don’t know where the lyrics are, I have them memorized. I’m sure someone can guide you to something
"genuflect in the direction of the container which holds the host? "
Again, you do not have to participate in this since you are not Catholic, but if you do- Yes, you genuflect toward the* tabernacle, t*he box in which the concecrated hosts are kept. If the red candle near it is lit, then there are consecrated hosts within

. Genuflection is a sign of reverance and resepct. If you happen to go to a church without the tabernacle in the front, behind the alter, then you make a profound bow toward the alter before entering the pew
