Hi I can answer 3 of your 4 questions based on my experience.
Once I applied for the wait-listed seat at lunch time, for a flight that was just after midnight that day, I called Singapore airlines as I had trouble doing this through Chrome, I was advised to do through safari, Anyway whilst on the phone I asked a few questions, She told me that it was very unlikely for the wait-list seat to be approved based on capacity of the flights, But within 4 minutes of getting off the phone I received an email to follow up confirm the booking, Which somehow came through even though I was told it was unlikely.
Points and payment WILL ONLY be deducted if you follow through with the booking confirmation, They give you a specific window or time-frame in which to complete the booking once your wait-listed seat has been approved. You can have several wait-listed seats.
Can you book a flights on waitlist and cancel it later, and does this cost you in points or money?
Yes you can cancel, by simply not confirming the seat, It costs you nothing in points or cash, If your wait list is approved and you don’t take them up on the offer they ask to call them and let them know, but I guess that can be up to your own discretion.
You can wait-list several seats on several different flights at any time.
How far in advance of the flight do they tell you if you have the seats?
This I’m unsure of, But they sent me an email within 4 minutes after wait-list application. May depend on how close to the departure date your flight is.
Can you ring their call centre and try and get waitlist confirmed?
I’ve spoke to them on the phone at the time and they said it was unlikely id get approval but it came through swiftly once I was off the phone. Maybe an option is to call them and ask if the probability of you getting your flight, They may give some feedback on to how full the plane is etc. But in my instance it wasn’t accurate.
Is it better to buy an economy seat and wait list an upgrade or should you just waitlist a full business class seat?
In terms of value for your points and dollar $
Is much better to try and get the full business class reward seat.
As only flexi fares can be upgraded with miles which are quite costly in cash value.
So stick to the full business class seats if you value what you get for your miles.