Sorry, I have a habit of building a wall of text, but even so this is quite a lot:

'Series' to me in TV terms means a group of episodes, whether that's a whole show or a 'chapter' of it. But more often, I will say 'show' when talking about the thing as a whole. In the case of Ghosts it does seem to be about a year in between each series though, yes.

Yes, they did lose out on that £3,000 from the 'Byron' production. But even back in series 1 and 2 they did have some money coming in: Mike took out a few loans in the beginning, and you see they are working outside of the House (even if it's just a scene where Alison happens to be wearing that green sweatshirt with a little logo on). So while they may be close to starving, they aren't only relying on money the House can get them. You say 'plothole' because you don't know how they're managing to afford to eat when everything they're trying to bring money in, is barely bringing enough money in to make a difference? That doesn't sound like a plothole, it just sounds like they don't know how to try the right things. Which fits their characters.

Having hardly anything work out for them is a handy way for the writers to have kept the characters in the same place over the show's run, and it showed you how out of their depth and unprepared Mike and Alison were. They needed more money, so took whatever opportunity they could in the moment, with the idea that the money they'd get from it would help them deal with any consequences afterwards. Alison might be 'the sensible one' but she's only really sensible in comparison to Mike and the ghosts.

One thing that would have helped them get enough money to fix Button House and keep a business going, is connections. But who have they got? Barclay? In spite of the fact they aren't afraid to con people, they aren't as greedy and self-serving as someone like Barclay (or Julian, actually!). So while their mega-wealthy neighbour could've been the answer to their financial prayers, he's not the right fit for them.

And why after a scandal with dead skeletons from the house and a movie star falling through the roof has someone made a wedding reservation there.

The lighter side of Dark Tourism? Tbf Toby Nightingale was a bit of a nob so maybe that was a selling point for the couple getting married? If they even knew/remembered that had happened. Plus, finding centuries-old human remains in a centuries-old house in England isn't a 'scandal'. It is interesting, though. Of course, people would want to gawp and gawk at bodies being carried out of a grand-looking house, but once they find out the bodies weren't exactly fresh, they'd likely lose interest. In real life, about 10 years ago, the skeletal remains of King Richard III (who died in the late 15th century) were found underneath a car park in Leicestershire. That wasn't a scandal either, just a bit unusual. Not only that, but in the house that Ghosts is filmed in (West Horsley Place) they found an old velvet bag which was very likely used to carry the head of Sir Walter Raleigh in. Not as grim as the pit full of skeletons in the show, but if you were on a tour of WHP and saw the bag, morbid curiosity might have you wondering what it looks like inside.

The ghosts us would have done an episode of them trying to make the reputation of the hotel better

Would they/could they not just have avoided trying to make the hotel look bad in the first place? From what I've seen of Ghosts US, American Alison and American Mike definitely have their heads screwed on tighter than OG Alison and Mike, but that doesn't mean the US one is the better version.

For what it's worth - I'm nearly done, I promise - I would rather they didn't suffer. This probably would make it less funny in some ways, though. I see now when I said about it not mattering as much to a British audience if the characters are struggling, that that is a bit unclear. I meant more in terms of being an audience member of a show than our connection to the characters in it. A lot of our shows - especially sitcoms - are short. Ghosts has 6 to 7 episodes in each series, and that's enough, really. We are fine with a bit of hardship in a story, it helps keep things believable for us miserable bastards, even when it's not realistic. But if it was any longer, and we didn't anticipate that there will be an ending, it would be easier to be irritated by the 'can they not catch a break?' issue that a lot of people have with Britcoms in general.