I can't remember/don't know how much water goes in since I will fill from the sink most of the way after I clean the fountain or when I first set it up. I will say that it holds quite a bit of water, and I check the level daily to see if I need to top it off with additional water. Maybe a quart or so? For the suction cups, they do stick to the bottom, and I wiggle it a little to unstick when I disassemble to clean. When I reassemble, I just press it back down and make sure I feel it stick - but I don't have to be too careful about drying the surface before re-sticking.

Around a gallon. Great product, go for it. I have 3 and my cats love it.

The filter comes on the plastic piece. You just throw away the old and put the whole new one in. I hope that helps!

Its pretty huge, as far as cat water fountains go. But it's advisable to have more than one water source (bowl, fountain, etc) for your cat(s). I have a water bowl next to my cats food bowl, as well as another bowl, placed at the far side of my kitchen, & my CATIT flower fountain is in the living room, with another water fountain placed about a yard away.from it.(one to the left & one to the right, of his little cat house.)

Yes, will work without filter

The manufacturer states it is BPA-Free and Lead-Free

I clean it out and change the water once a week. My cats are always drinking out of it so I have to add water after a few days. It's a great fountain! And yes, the filter is to be changed once a month. It'll need it!

I think it keeps running - it gets very noisy with splash sounds when the water is low which is a reminder to fill it back up. 

You CAN adjust the flow. On the water pump mechanism there is a very small lever that slides from left to right. One side shows (-) and the other side (+). Try that out!