I believe you are misunderstanding the intent of the form 966. This form just tells the IRS that you have adopted a plan of liquidation.  It doesn't mean that it starts the timing of the final return.

Liquidations are generally governed by Section 331 and 336.  Liquidations can take several years in some complicated situations.  Your time frame for the timing of your final tax return will be governed by the final liquidating distribution.  Keep in mind, that liquidating distributions are not reported on the K-1 as normal distributions.  They are reported on form 1099-DIV.