Hi, and welcome!
Rachel answered a lot of your questions, but I wanted to answer one more. If you made a card that was designed by someone else, you can certainly post it! Just note in the comments that it was designed by a demonstrator (or teacher, whatever) and if you know their name, post it.
We all copy eachothers stuff here, the code of ethics is just to let everyone know that it was inspired by something else!
Enjoy, and again, welcome!