Popchyk My DM was exactly the same when my father died. Luckily he was pretty organised (and as he had been terminally ill, time to plan), so had let me know all about his financial affairs. In practice it meant I managed my mum's finances for a few months and she gradually took them over - she's now more organised than even I am.
The targeting after my dad died was horrific. I was on maternity leave so spent a lot of time at my mum's house and basically ran interference, but it was a very very difficult time, and as you say, made a stressful time even worse.
She also now phones me / DH whenever she gets a letter / email that she's not sure about - there's a load of things she thinks might be a scam, but she's not good at working out what's genuine...DH deals with that mostly.
DH and I don't split things evenly, but he has arthritis in hands and other joints which do limit what he can comfortably do, and he does do what is reasonable, including a chunk of the mental work. eg Christmas, I might go out and actually buy stuff / do the cooking, but he's the one who looks into Christmas presents, thinks about what people want, discusses what we'll have for dinner, talks to family about where we'll be / who's coming over etc etc - in consultation with me, but it's his task to deal with.