sorry about your loss
there is no December 16th rule for mutual fund distributions. a specific mutual fund or mutual fund family can pay out on 12/16 and on no other date for the year, but the 1099's will cover the full calendar year. the owner on the date of the distribution is the one to report it on their tax return
don't know where you got your info from, but i can guarantee you many mutual funds pay out on multiple dates. some even monthly. as a matter of fact Mutual Funds can make distributions in the following year that are taxable in the current year
if you mother's SSN is on the 1099 but some or all of the distributions were after her death. you'll need to report the 1099 amounts on her final return. for those after her death, TT can not handle negative amounts for things like dividends, interest or capital gains (negative proceeds) the only way TT can handle this is through using the nominee boxes. the recipient info is for your use only it does not appear on forms. TT does a poor job of this in many cases. Even if not required, if might be a good idea to file a final return for her.
you could then report the distributions pro rata on the beneficiaries return
technically this is supposed to be done but in practice I've never seen it.
Nominee/middleman returns. Generally, if you receive
a Form 1099 for amounts that actually belong to another
person, you are considered a nominee recipient. You
must file a Form 1099 with the IRS (the same type of Form
1099 you received) for each of the other owners showing
the amounts allocable to each. You also must furnish a
Form 1099 to each of the other owners. File the new Form
1099 with Form 1096 with the Internal Revenue Service
Center for your area. On each new Form 1099, list
yourself as the “payer” and the other owner as the
“recipient.” On Form 1096, list yourself as the “Filer.” A
spouse is not required to file a nominee return to show
amounts owned by the other spouse. The nominee, not
the original payer, is responsible for filing the subsequent
Forms 1099 to show the amount allocable to each owner
