I’m sorry I don’t know what goes into a Singapore sling. I had a white wine there and I know that it was gluten-free. There are many gluten-free options on the menu. Just ask the waiter and he will help you.

Yes, the Singapore Sling is GF and my wife (who is gluten intolerant) was able to eat in the courtyard bar area.

Yes you will get butler service. We had dinner (indian) in the Tiffin room which doubles for breakfast and really didn't seem that great but expensive. There are other restaurants even pricier in Raffles such as Mdme Le pic (3star) or the steak restaurant again expensive but likely much more well worth it. Afternoon tea is good, don't worry about the writers bar (booking) as they don't seem to bother or with the recommended dress code Teh long bar is a great experience but remember to use you raffles (room) card just walk passed the (hour long) queue and show it - you will get straight in. Courtyard does good food and drinks as does the coffee shop adjacent to it Remember this is Raffles - you will pay a premium but believe me it really is worth it Despite being able to get 3/4 days at the Fairmont over the road for the price of one night I would still do Raffles - we're going back

Suit no needed, but better not wear slipper / shorts . Smart casual would be fine.