1.) 15% is only for certain divisions. From what I know, 15% can be achieved from the Endless.com shoe/accessory division of Amazon. Everything else is tiered from 4%-8.5%
2.) 24 hours
3.) I assume a new affiliate link will replace your cookied one. After 24 hours, it doesn't matter. Most affiliates find that if they don't purchase it in 24 hours, they probably won't purchase anyway. The real way people rack up the commissions is when people buy a ton of items at once.
I also believe that this affiliate program is an uphill battle. Short 24 hour cookie, tiny commissions on tons of small ticket items. You need to push major volume if you want to break 5k/month.