This theory is more speculation at this point for one reason. THERE IS NO EXACT WAY TO TELL ANYTHING WITH AN SHOW BASED ON SHORTS!!!!! Now that I say that, I will go over various matches I did find riddled throughout the show that give a surprisingly different take on how Eustace and Muriel could have met, married, and why they live in "The Middle of Nowhere". The order I will place each of these events we've been given in the most likely chronological order they happened rather than by looking at each episode they happen because of the sheer number of episodes given for the show. Year Zero being given as the birth of each character, and an approximate year for every event we can confirm until reaching speculation.
Year Zero- Birth of Muriel and Eustace
Year 3- Muriel has an extended terrible twos (as seen when she was forced back to aroundthat age due to a tornado)
Eustace brother is born (Speculated that once Eustace was 3 his sibling was born and he was shunned because the sibling was his better) and first time he saw Muriel (speculated on his reaction to Baby Muriel, he seemed to wholeheartedly know this Muriel since he didn't freak out at once and even a hallucination would be likely to have him scream at the child)
Year 5 - Muriel and Eustace begin school (not much here outside regular school code)
Year 8 - Eustace brother begins school, and Eustace competes with him despite knowing he'll lose (speculation over the awards shown in Eustace Mother's house and the time it would take to acquire so many in natural circumstances)
Muriel begins singing (We are shown she has learned "Little Buttercup" since a young age and this works best in relation to a few other events in the coming years)
Year 12 - Eustace father finally shows he only cares for his work and begins having Eustace and his brother work under him (most farmers have done this, but it will also show how Eustace got his house with infertile ground later in life)
Muriel begins skipping school to be with Eustace (Speculation on their dynamic being natural to the point it seems like they've done it since childhood and it leads to something else on speculation that could be a big event)
Year 16 - Eustace father comes down with a sickness and they get a support animal, specifically a dog (speculation related to the state of the shoes shown in Eustace Mother's house, also to the reaction of Eustace whenever these shoes are mentioned. As well as Eustace mother's reaction to Courage alongside Eustace own reaction)
Year 16 - Muriel begins playing sitar to help Eustace family (again, speculation as to Muriel's skill and a likely way she would act)
Year 18 - The Bagg Family realizes where the money is coming from and Eustace father dies (speculation, but if this is right, it could explain why Eustace Mother hates Muriel. It could be a belief that her husband died as a result of Muriel's actions)
Muriel and Eustace try to elope rather than stay due to Eustace family, get caught as they are leaving, and Muriel lets Eustace mother know how she feels (this is an odd one, but there is an odd connection I found between a few historical musicians and Muriel's own treatment)
Year 19 - Eustace gets a letter from his mother on his inheritance. He now owns the farm shown in the show. (I believe this farm would have been given to his brother so he could change it, but in the end his brother took to a different lifestyle after multiple successful ventures during his final years in school.)
Year 21- Eustace receives the first of many letters from his brother regarding the exploits he's had. (A constant source of stress for Eustace is centered on how his brother has had such an eventful life while his own has been dismal, this had to start pretty early for his reactions later in life to be so exaggerated. Another moment may also make this the time Eustace receives the hat as a gift. Albeit as a final gift from his father with the hidden key to his brother's own growing horde of gold and treasure.)
Year 35- Eustace begins visiting his mother once again. His mother clearly favors his brother at this point. And Muriel goes once before she resigns herself to having walks around the city to pass the time (Again, not much said outside Eustace mother hating Muriel and it being slightly returned because Muriel never visits for odd some reason)
Year 55- Muriel finds Courage while walking around during one of Eustace visits and keeps him since Eustace and her haven't had kids. Eustace begins to see the dog as a rival, but never going too far since he cares for Muriel. (Again, not much outside regular occurence, but the episode showing baby Courage has one thing that doesn't seem likely to put Courage past the age of 14, and Muriel most likely near 70 when we see her. That thing is the people seen in the background. A few of the adult deviants shown in the city actually appear as children if you look closely enough.)
Year 56- Eustace takes Courage to his mother's house for the first time. His mother immediately takes a liking to Courage after the dog that helped with her husband, and gives Eustace a worse time thanks to the memories that resurface of those days. (I know, this doesn't sound right, but if you think along the lines that Courage has been known to be compared to Old Yeller, then it works.)
Year 60- Eustace brother is confirmed dead. (There have been various years given to the comparison of the brother's death, but the average is 9 years when taken all together. And again, time doesn't pass in the show, since it's shorts, so all this taken when matching everything together).
That's it. After that, I couldn't find a lot of times related or directed to any years that are confirmed in the series. All this is pure speculation based on what we're given. So, again, if I'm dead wrong, it's just my thoughts for this one. For all I know it could be like the majority of theories where they're in hell, purgatory, or something else entirely. All I know is I agree with the Chinese guy whenever I'm in my car, riding my bike, or walking down the street.
"Watch where you're going, you fu!"
Seriously, though, this series is going to be increasing like crazy after October is done. October 6th, 2020 is the finale and thanks for Son of Time, go read that for something to keep your thirst satisfied. And I'm taking a break for all of October, see you guys in November for a special month of Ben 10 theories!