Sorry you are going though all this at once....
There is no issue with being both executor and beneficiary of a will, in fact it really makes everything simple, so long as there are no credible challenges to the terms of the will. A child would inherit, before siblings, if there were no will so all good. I was both, and prepared the forms for probate then had the solicitors review them, just to make sure I had not made mistakes, before applying for probate then distributing the estate. It takes a bit of effort to execute a will, I cannot imagine that anyone would not be glad to be relieved of the burden, especially if they were not beneficiaries. In the same way, if they are not all that keen,they could give the matter little priority or even drag their heels, so that everything could take ages and ages.
To make any changes to a will, the person must be judged capable of making these changes. If they are, then crack on, if not, best not make any changes incas it calls the whole thing into question.
Delaying paying in the check seems unnecessarily complicated for no benefits. Pay it in, and keep a note that it was a gift made in contemplation of marriage, so long as the marriage goes ahead, you are all good. Sorry to have to point this out, but the bank would have to take notice of the date on the death certificate and would probably freeze the account. If the cheque was uncashed, that would add it to the debts of the estate. No biggie, but i see no benefit in delaying paying it in.
All the best with finding your way though all this OK.