If you avoid prepaid rates, most chain hotels will let you leave early, at worst for an early check out fee of about US$25-50. It sounds like you might be looking at third party booking sites, which are almost exclusively prepaid/advance purchase rates.

For your locational criteria, look at the (approximate name) Grand Copthorne Millennium. It's along the river, near Clark Quay but in a quieter area that's somewhat of a newer upscale residential area. You can walk on paths along the river to the action closer to Clark Quay. There's no really close MRT station, but you can walk to the far end of Orchard easily or it's a very quick taxi ride. There are several other hotels in the immediate area that are slightly cheaper.

If you search, there's a stunning boutique hotel in Chinatown that some folks here frequent. The rooms are small (long stay?) and it might be a bit expensive. It doesn't have a pool and might not have a real restaurant on premises.

IIRC there's a Novotel right at Clark Quay that looks nice and new, although not especially fancy.

Depending on how full the hotels are for your dates, look at the more modest chains near Orchard: Hilton, Marriott, Sheraton (but this one can get some bad reviews and especially seems to have mildew problems in some rooms) etc. (The Hyatt is probably too expensive and St Regis is definitely expensive. IIRC there's a slightly less know chain property--I forget the name--near the St Regis that looks pretty good, located almost at the bus stop for the Sentosa express service around the corner from the StR.)

For this length of stay, how about an apartment hotel where you would get a small fully equipped efficiency, studio or one bedroom with mid-range hotel services? Singapore has lots of these, usually local chains, as well as the longer term serviced apartment set-ups.