@TheGlossy As Betty says, your main priority right now, is to get the pay situation sorted out. Make sure you show attachments of the handbook, with highlighted areas and if your manager put anything in writing about taking time off, show that too.
It's irrelevant that the Manager has now left, at the time, they were the Manager in charge of your team. Don't let them fob you off at all. If you need to pull your belt in financially (because as Betty says, they might fix things, but put it onto next month's wages instead of sending it now), then have a look at all your bills (things like gas/electric/council tax type stuff) and contact each of them to see if they offer any sort of payment holiday scheme.
I am very much feeling for you with regards to the family situation. So let's break this down a bit as I feel you are very probably overwhelmed at the moment.
For you, it's still incredibly early days of grieving. Remember that you have had a difficult situation familywise, so your way of grieving may look completely different to how others would do so. You aren't gathered together in a living room, showing family albums, watching videos together and letting some tears out.
There is no correct or incorrect way to grieve, it has no time limit, no use by date, it's going to work it's own way through and if you don't take a little bit of time for yourself, it could well creep up on you and whack you big time in a few weeks, or months, making you ill. So you could do with some peace and quiet to process everything, so your mind, heart and body can all breath and think.
Your family (Mum and siblings) are clearly designating you as their Driver (of the estate paperwork) so I'm going to tell you, there are also taxi's, buses, trains and planes out there ok

and you can hang your keys up, without feeling any guilt at all.
I want you to do one thing (and frankly and honestly, you don't even owe this) but I know that you are a decent person, you do have a heart and I don't want you to feel sad, bad or in pain because you feel you didn't do something.
Find a Solicitor, in their area (not yours) who covers family law, wills and probate. That is the section of law needed. Now this person will take a fee for their time, their services and so on, but you know what? You are NOT covering it. The family, who are receiving a roof over their heads, food in their tums, money should Mother pass and leave it to them (and she may not do that either) they can all get together and assist her with the help of a paid professional.
Should you screw up, forget something, not do it right, you could potentially cost them a lot of money and stress, I promise you, you don't want that, so for your own safety and health, I'm asking you to please arrange the Solicitor numbers, then pass it to the family and let them do the ringing round, booking and working with to sort the financial stuff out.
You can then removed yourself from that mega pile of steaming stress and pain and take very good care of yourself. How does that sound to you? x