To answer your question:

1>, No there is not a rate like 2-1 or 3-2 available in US like what they do in UK.
2>, Yes there may be seasonal rate open by US marketing arm of IHG provide that they feel that it is one urge to open such rate to damage their ADR performance (e.g. Double Dip, re-recession, 9-22[oops has to be next year]...)
3>, Individual hotel may do such promotion occasionally like IC Tampa, which they will specify in their own website. It requires a lot of search and disappointment.
4>, The common special rate in US is Pizza Delivery, Free Movie, Gift Cards...They beleive in add-value rather than slash management performance.