Legal advice on Retirement accounts and estate planning

I have a great relationship with my step-dad of over 60 years. He has four biological grandchildren with my deceased Mom. Do they have any rights that I do not have in the case of the death of my step-dad? He is 92, I am 72.

My husband convinced me of signing a postnup agreement in 2015 in order for us to avoid a divorce, I really didn’t understand it but, to save my marring I signed it and we had it notarized but no witness. I don’t have a copy of it and his is in a bank box.

I had to resign from my retail job for medical reasons [caused by that job], but I received a communication from the 401k company a couple months later. I was informed that as my balance was under the plan minimum of $5000 (today my account was still only $608), I would be receiving an automatic distribution minus federal and state taxes. The last day to change my mind about how I wanted to receive the pay out [check, ira, etc] was 12/28/18 at 4 pm. On the 29th I was sent an email that said, "We previously sent you a notice informing you that your vested account balance was less than the minimum balance requirement for your previous employer's retirement plan. Upon subsequent evaluation, it has been determined that your vested account balance is no longer subject to an automatic distribution and will not be distributed without your consent at this time." I don't understand, my account actually went down $30 since the first notice, what happened? Why am I not receiving my money? I'm only 30, this was my first 401k and it's just under a year old.

I'm trying to negotiate a settlement with my husband and know I'm entitled to half of the 401k retirement plan but I really don't want to take a 10% hit with the IRS. I want to know if I can leave my portion where it's at and still be entitled to that half when I retire? Also, if I'm not able to do that, will I still be subject to that 10% penalty on a home purchase or if I roll it over into an IRA? Thank you