You need to move the floors back to the BACKGROUND (FLOOR 1) layer too. The corridor tool operates on both floors and walls, and expects both to be correct for it to recognize an existing entity in your map as a dungeon room/corridor to work with.

Check this FAQ topic:

For symbols, you can just hit the options button on the top of the symbol catalog option and turn off the option that symbols choose their own sheet.

That said, I recommend you don't do this. This feature is a huge time saver when mapping, and really helpful to ensure things go to the right sheet. It may sound like having more manual control is a good idea, but in practice you're just setting yourself up for much more work, and a lot of symbols on the wrong sheet because you forget to change it.

I recommend learning to use the feature in your workflow instead of turning it off. See this FAQ entry for some information:

Easiest is to just hide the sheet you don't need, this ensures things doesn't get in the way. You can do more complex selections too, look at this article for more about that:

The break in the wall is a permanent edit (In most cases it will divide the wall into two separate parts.)

If you change your mind right after breaking it, you can always use undo (ctrl+Z), but if you have done other edits in the meantime and undo isn't appropriate, you need to manually edit the wall. In most cases, just using the node edit tool to bring the end node of one end over to the other works good enough, although it might end up leaving a line at the break point due to the fills not lining up seamlessly.

Other options include just drawing a small wall-piece into the hole, or just simply delete the existing wall and just redraw it. That's usually quick using a wall drawing tool.