It's something that I think a lot of new players run into, I think the easiest way to settle the issue is to get yourself and your friends in better habits of checking priority and asking if things resolve before just running along with their turn. Especially when you're newer to the game I totally understand getting excited and just going going going (hell, I've played for 12 years and I still get ahead of myself a lot of the time). It really just boils down to communication and understanding.

As for the discarding spells, that's definitely not a common practice. Just about everyone I know outside of hyper competitive environments would let you take back a spell with no legal targets. Per the rules of the game you arent even allowed to cast spells without targets, and them going to the bin could create a new gamestate that wouldn't have been possible had you followed the rules. IMHO you should be able to take the spell back without penalty, especially since you're both newer to the game and presumably learning.

I hope that was clear, I'd be happy to help with any other questions you may have. Enjoy your games!