Why repeat it ?
The issue of mistake or not...
....could be relevant in some legal proceeding,
but likely not in convincing hotel to honor.

One avenue to pursue now is to bypass
the Singapore agent sending the e-mails,
identify a higher-up in the decision chain.
Write polite but firm correspondence
requesting they honor the confirmed rate.

Perhaps someone at IHG HQ,
but I will start here:

General Manager
Stephane Varoquier
[email protected]