However, Code Section 72(t)(2)(H)(vi)(IV) provides,"Any qualified birth or adoption shall be treated as meeting the requirements of Sections 401(k)(2)(B)(i)..." The cross-referenced section lists the permissible distribution events under 401(k) plans.
It does not matter if you were employed on 12/31 or not. A SEP IRA can not have a last-day-of-the-year employment requirement. As an eligible participant it does not matter when you separate employment. The employer must base your 2019 contribution on your 2019 compensation. You will even be entitled to a contribution for 2020 based on the amount of 2020 compensation.
Depends - check to see how the form reads. Also need to know when the contingent beneficiary died in relation to the primary beneficiary. I could see situation where the lone surviving contingent beneficiary gets it all and I could see a situation where the deceased contingent beneficiary's estate is entitled to his/her half.