So the good thing about Singapore Airlines is that you are allowed to reserve an award seat, but not ticket it yet (i.e. it's yours, but you haven't paid for it yet).The May 15th deadline she speaks of is the ticketing deadline. By this date, if you do not ticket it (i.e. pay for it using your miles), the seat will be released and it is no longer yours. If you were to ticket it, miles will be taken from you, and all the Krisflyer redemption ticket fees will kick in after that. For example, if you wish to change the date of your flight, that's $25. If you wish to cancel it, that's $75. In your case, if Business becomes available and you wish to change your ticket to Business Class, that's $75.

The next good thing is that in general, you can call in to ask for them to extend your ticketing deadline. I can't tell you how near or far to your flight date the new ticketing deadline will be. Generally you take what you get, and nearer the new deadline, you can call in again to try to keep extending it. During this time, you can ask them to also send a chaser to clear your Business Class waitlist. And unfortunately, all these can only be done over the phone.

As long as the deadline keeps getting extended, the Premium Econ seat is still reserved for you and anytime you decide to give up on the Business Class waitlist, you can issue the Premium Econ ticket and the miles will be deducted from your account. However, I would be in no hurry to do that since the seat is yours anyway as long as you don't cross the deadline. Sometimes I have extended and extended and finally only issued the ticket one day before my flight.

Good luck!