Hello
Well done Chris. Yes it is a double sided soaker.
Perfectly waterproof.
They're not very pretty, I know, but often, it's the only way to join two roofs that have different type coverings.
Actually, in the pic, it looks as though the coverings are the same, so it could have been omitted if so.
I guess he was trying to separate your roof from his, in case of any future arguments.
It's not a pipe. Nothing flows down the hole.
Looks like a good job. Nothing to worry about.