1) Early bird tickets for business conferences seem very standard to me. I've always done them for all of our events.
2) Totally agree that it's awkward if people have purchased full price tickets. I guess you could either hope they don't notice or offer them some additional value as part of their full price ticket (although I'd still be miffed if I'd paid full price and others were getting 25% off)
3) Yes probably.
Other ideas include discounts for colleagues when buying more than one ticket to encourage people not only to register but to bring a friend (but has a financial knock on) or some sort of incentive prize as you suggest.
Another idea is to get partners, sponsors, speakers involved in a massive campaign to sell tickets, maybe offering their connections a discount (less awkward) or giving them a kick back for every ticket sold.
I've been there myself with sluggish ticket sales. Good luck