You don’t have to fake grief or make speeches about anything. TBH ( I hope this doesn’t upset you ) it sounds like your other parent is a rather self absorbed person, because they have never noticed that your other parent is not very good to you and doesn’t love you.
The parent who will be widowed is not going to change now and start thinking of anyone apart from themselves. So they won’t be expecting you to show any feelings, they won’t even notice.
If it’s been all about them and their husband / wife for ? 40 or 50 years , it’s not going to change now. It will continue to be all about them.
So you don’t have to worry about faking anything. If random people ask you how you are feeling / coping etc , just have a a few stock sentences / platitudes eg
yes even though they were unwell for so long it’s always a shock at the end isn’t it?
yes it’s hard for mum / dad - do you know they were married for X years .
as you know they have been ill / in hospital / in a nursing home / for a while so it was hard for mum / dad to see then so unwell / disabled / in pain.
yes it’s been a busy time winding up the estate / selling the house / organising the funeral / supporting mum / dad .
I’m now an expert in saying things that are not a lie but are not the truth , which would be “ Actually they are evil and abused me as a child, wrecked my siblings marriage and abused their grandchild so I’m glad they are dead”.