Really mean things people do to save money.

Perhaps your MiL grew up in the Hungry Thirties daddima. Old habits die hard.
My dad used to talk about how (in the 30s) only my grandpa would have an egg on his toast for breakfast. The rest of them had dripping on theirs, if there was any.
And please don't spit

I canmake a fluffy omelette with one egg, if it is a large one but usually choose to use 2 for me and 3 for DH.
Does anybody else still do that think of giving the "man" more protein as if he were going down t'pit or ploughing the 5-acre field before tea?

I'm willing to bet that the OP would still have disliked the mil just as much if she had been a spendthrift!

OMG 4 eggs to feed 4 people an omelette each.

When I stayed as a child for a few days with my grandmother she would put the mince on for lunch about 8.30am, it would be "ready" for 12 mid day and it was grey and disgusting with cabbage and potatoes which were slop.

Shepherds pie always has carrots and a tin of baked beans in this house, ructions if left out

PS I love hot tongue with Madeira sauce!

I won't tell you what I have added to mince to "spin it out" for shepherd's pie in the past, well actually I will. Grated carrot, onions, toasted breadcrumbs (OK just a smidgen) or else a little stuffing mix. I reckoned I could do a shepherd's pie for well under 50p per person back in the day.
If stuffing cabbage or vine leaves or red or green peppers I always add rice. So
#justcallmescrooge

MIL dried out teabags to reuse them.
DH's godmother was infamous for having mince every day for lunch - boiled, grey and made your teeth 'bounce', with boiled potatoes and cabbage.
Christmas Day was boiled tongue....... I don't think anyone ever went back for a second meal!

Might be she had a 'frugal' upbringing.

Words fail me.