Sorry for your loss...
I think you are combining 'apples and oranges'
first there is no gift tax - nor is there a reporting requirements about a 'gift' nor does your wife's passing limit your contribution to a 529 in either the dollar amount or the time urgency to make a contribution.
the 529 plan remains in your name and your child is a beneficiary of the plan; you are not giving up control of the money to the child. So there is no 'gift' occurring because the account remains in your name and not the child's.
also, there is no $15,000 limit (but see below) so easiest if you establish the account in your name only and contribute what you think is necessary and within any limits your state plan defines.
from IRS publication 970, page 52 (top right of the page)
How Much Can You Contribute?
Contributions to a QTP on behalf of any beneficiary can't be more than the amount necessary to provide for the qualified education expenses of the beneficiary. There are no income restrictions on the individual contributors.
