You mention they still need windows for an accounting software, did you decide to go with another PC brand then and might I wonder if these issues would be different with another brand?
Overall the experience makes me think that it’s going to ultimately be a windows 11 issue (this is apparently the ultimate requirement for the accounting package) - unless the way another OEM configures manages to avoid the Microsoft account, advert pushing and O365 direct to New Outlook?
(As a side note, I’ve previously had them using parallels on a Mac. It worked but it was hard work. My parents are both brushing 70. They’re familiar with Outlook - when buttons change it blows their mind. I’m sure everyone in here has had to deal with parental tech support - now try managing it when you’re 900km away and they’re using an operating system you’re not even familiar with any more)
Maybe sticking with an older version of that software that doesn't need windows 11 might be the best option if possible.