Can the beneficiary (spouse) waive the QPSA after the death of the participant (Before RBD)? If so, how long does the spouse have to waive?

The law and regulations permit a spouse beneficiary to waive the QPSA after the death of the participant. However, the plan documents or the forms being used might prohibit it. You need to look over the relevant sections of the document and the language in the forms to see what the answer is for a particular plan.

I'm assuming your second question was intended to be "If so, how long does a spouse have to waive?" Assuming the plan does allow the spouse to waive the QPSA benefit after the death of the participant, I would think the spouse would be able to waive the QPSA benefit anytime prior to the time the death benefits are required to be distributed from the plan. Here again, there could be something in the plan document or forms that might say otherwise.

i am concerned after reading my plan document (ppd/corbel). It seems as though they cant waive after the participants death. i need to review it again.

See Section 6.04© of the PPD/Corbel Basic Plan Document.