Recently, I had roughly the same arrival/departure times that you had in Singapore. I wandered around parts of the city until the last train left for Changi, and then somehow killed time in T3 and its basement floors. I was denied access to any *A lounge because MI flights apparently don't allow one to reap the benefits of *G, alas you will have a more tolerable experience (I hope/reckon) with your onwards SQ biz ticket.