Standby tickets at the venue fixes this. Similar to airplane tickets, which can be oversold, standby tickets accommodate those who really need to use a ticketed service.

I doubt there would be that many no-shows, demand for tickets far outweighs supply (usually). People who were making causal purchasing choices hitherto may be more cautious as a result, that's all.

If you restrict refunds to be permissible only until a certain point before the event, that will create a window in which unsuccessfully scalped tickets are released back into the pool for general public, bypassing scalpers. If scalpers choose to hold onto the tickets beyond the point, they wouldn't really be able to liquidate. I'd say that we shouldn't let perfect be the enemy of good, with regards to refunds.