Can I use a Time Capsule as a backup and connect an external drive to it...

Same result, yes. The backup will be aborted, the backup files shouldn't be more damaged than that a simple checkup during the next backup will fix it. But make a habit out of checking if Time Machine is running, because I've read about people who have had corrupted backups due to the connection going down.

 

Well, the exact same results but possibly slower, since wifi is gonna be slower than a USB connection. But whether that makes a difference depends on what you're backing up and otherwise moving around.

And you could set up your laptop to backup to your TM backup, wherever it is, manually. I just make a habit of doing that when I hook it up to recharge over night. If you use some of the versioning built into some applications you maintain at least some backups while you're working. There might be a way to enable local TM backups with TM on the laptop while it's attached to your LAN but without writing to the TC, but I’m not sure.