HelloNoora
Kilomech
I don't think any of those things matter with babies. If you want to walk around your house nude with your three-month old watching then so be it.
It gets weird once they get a little older though, like age 4 and up. Same with multi-sibling bathing or bathing with parents. It's okay when they're younger, but when they get older it gets a little strange.
Of course, I see no reason to let even a toddler see you pee. That would make me uncomfortable. A newborn is no big deal, but a kid who can walk, point and ask questions? Awkward.
Our entire family demo'ed "going potty" for the ones not yet potty trained. Mostly I ask that we close the door when using the toilet. However bathing or changing together is simply not a problem unless people have a screwed up idea of the human body and human sexuality.
When I was being potty trained, I don't recall ever being confused at the concept of using the toilet without being showed how first. My mom supervised me, of course and she double-checked to make sure everything went...well, but I don't remember her sitting on the toilet and saying, "This is how it's done." I mean, you just sit and pee/poop right? The hard part, I would assume based on experience, would be in the wiping (IE, front to back, make sure you get it all, etc). Not that my mom closed the bathroom door (or closes the bathroom door) when she uses the toilet, though. She has no shame.
But I figure that with adequate supervision and substantial appraisal, a child should have no problem knowing how to use a toilet. Accidents will happen, of course, but as long as they understand it, I don't think it'll be too terribly difficult. I wet my pants often when I was very little, even when I knew how to use the toilet. I just had terrible bladder control and was too shy to ask where the restroom was often.