Also, dementia is exhausting for the sufferer. My father is constantly drained, and if something extra stresses or tires him, his dementia takes a steep dive. I can always tell if he has an infection, because his dementia symptoms deteriorate significantly. I'm guessing that his poor, diseased brain is having to work extra hard to do the simplest tasks for him, so any extra burden tips him over the edge.
As far as 'filling in the gaps' is concerned, I think that the more intelligent the person is (and my father is extremely intelligent), the better they become at inventing a narrative to explain the gaps. My father hid his increasing deafness during sixty years of a high-powered professional career by filling in the gaps of a conversation he missed if someone looked away as they spoke (he lip-reads), or if there was any background noise, so filling in gaps of memory is the natural thing for him to do.
Sadly, that often leads to problems. His memory of my mum's death is that she broke her back and died from her injuries. She did break her back, but had recovered and lived well for 8 months before dying of a strangulating hernia (post-op). He also believes that the special music player I bought him at enormous cost (designed for dementia sufferers) was actually something the builders left behind and that he stole it from them. This is hilarious, because we haven't had any builders in, and neither had he and Mum, not in the last thirty+ years, and he keeps this secret from me because he knows I'd be cross with him for stealing. Luckily, he told my mother-in-law this in confidence when she complimented him on it. And isn't it lucky that the builders loved exactly the same operas, and musical theatre, and religious ballads that he enjoys so much?
The good things: He used to have a terrible temper and a short fuse. He was very quick with his fists, having been one of seven brothers in a single parent family growing up in the war years in poverty; these days, he has no temper at all, and is totally placid and understanding. Some dementia suffers head in the opposite direction. A friend's mother, who was a sweet-natured and strait-laced old dear before the dementia now swears like a trooper and lashes out violently.
Dad keeps a diary. He records, in his shaky hand, everything he sees throughout the day. During the lockdown, he entered the number of cars in the yard several times a day every day, despite the fact that number never changed (we have five adults living here, and three cars between us).
He sometimes notes down things he's hallucinated, like a conversation he had with a chap in the fields in front of the house. The man told him they were his fields and he was grazing cattle in them, then sheep, then he might try growing wheat. That's our field he's talking about, and no one can't get in it as the gates are all chained and padlocked (largely to keep Dad from wandering off, which he tries to do often). We do have a ghost here, so perhaps he's been chatting to him!
For Dad, my mum is sometimes still alive. She talks to him and holds his hand in bed at night. He knows she's dead, but I think it's a bit like the radio. He tries to keep her secret because he thinks it will upset me, but he tells Fraser and my mother-in-law about the things she tells him.
The hallucinations (or alternative narratives) can be a problem on occasion. Not only because sometimes he's convinced that one of us has done something bad to him. Like stealing his garden furniture (the furniture he's sitting on at the time), or transferring all his lifesavings to my son's bank account (I wouldn't trust Patrick with a tenner for the messages, never mind someone else's money!), but because they can be more real to him than reality. I've come in to find him sweating and crying because he was back in Malaya, in the middle of a river ambush where his best friend took a bullet meant for him and bled out in the muddy water. This is more than a memory; it's real to him. He feels the heat, hears the noise, smells the blood. Those times are especially hard on him, and, by extension, on the rest of us. We all love him.