So what happened to us then?
Probably nothing.
Demons are the only answer, unless you believe in Ghosts and life after death etc.
Well, ghosts, goblins, pixies, aliens, and telekinesis caused by your brother's mutant mental powers are all at least as likely as demons. But far more likely is that, you, as a child, with superstitious parents were influenced by their beliefs and behaviour. You remember strange events which probably had completely mundane explanations or didn't happen at all. Or you all ate too many of those funny mushrooms that were growing in your garden.
I don't, because unless something else makes more sense to me, I shall continue to believe in the bible. I do however believe in Demons, but only because I had first hand experience of them, the bowl and the lady were only two things that happened, there were a lot of other stuff that I'd really rather forget.
So the only reason you believe in demons is because you had an experience that you can't explain? Why pick demons above any other imaginary entities?
And it was mighty odd how they cleared off when the cross was removed.
Not really. It was part of the whole belief structure that there would be some sort of talisman involved. When this was discovered and removed, the "events" stopped. If they hadn't, of course, the search would have continued for some other "demonised" object to blame. But the cross was probably a suitable enough symbol that it put your parents' minds to rest.
Why does an upturned Cross have such power in Black Magic?
Because it's a subversion of a symbol mostly associated with Christianity. Inverting it is blasphemous and associated with the devil. Of course, in reality, it has no power, any more than a right-way-up cross does. They're symbols of belief systems, nothing more.