Ah, a chance to stretch the old theory brain cell again!
1.1: Trevor's Training - Chemistry set - Flat tyre
I imagine that at the end of Trevor's training, the crew go downto Bella's after Elvis (again) messes up the lunch. As they enter the café, the stage lights move to focus on Norman's bedroom window and we hear Dilys complaining to Norman about making his famous stink bombs with his chemistry set.
After the chemistry set fire is put out, Dilys takes Norman to Newtown to shop for some replacement items lost in the fire. The twins ask to tag along and they do. There is then a 'time skip' to them walking home from Newtown to get a cheaper bus ride.
1.2: Camping - Treasure Hunt - Pontypandy Birthday
After the failed attempt at camping off Pandy Lane, the Twins are pitched up in Sam's back garden. Sam is tidying up and is moving his metal detector to his shed. The twins ask what it is and Sam says he'll show them tomorrow.
After fixing the damaged water pipe and locating Bella's keys, the metal detector beeps one last time before leaving the park. Carefully (this time) James moved the dirt out of the way to reveal a long forgotten plaque. Cleaning the plaque revealed the text Pontypandy (or Pontypandy Park): Est. xx-xx-xxxx . And it is discovered that the village (or the park) was founded / built say 100 years ago, and the anniversary is coming up in a few days, so a big celebration is planned.
2.1: All in a good cause - Trevor's tragedy - Bentley
I think Trevor's Tragedy could be based on the mini world book 'Sam's night watch' where Trevor gets locked out of his house and gets stuck in the window trying to get in. With that in mind:
After being relieved with Dilys rescuing the raffle ticket money from Bella's bonfire, the crew are back at the station. Steele informs Sam that he is on nightwatch duty for the next few days, and should go home and get some sleep. He bumps into the twins on the way and tells them not to knock on the door as he needs to be well rested for the night shift.
After almost forgetting to set his alarm clock for another nightwatch, Sam wondered if he could invent something that would wake him up on time for his shift and have breakfast ready for him. He then wondered if this invention might be able to do other things, like go shopping. 'it could be like having a butler' he thought. The next free time he had spare, he set to work building 'Bentley' the butler robot!
2.2: Norman's Pitfall - Bella's BBQ
I unfortunatelly can't suggest a connection for an existing storyline for Bella's BBQ, but I don't have to for this one.
When first having the picnic with Sarah & James, Bella mentions that she loved cooking on barbecues before she opened the café. The twins said they would love to have a bbq sometime with her, and Norman overhears them while chocolate cream eclair fishing. At the end of the story, after bringing Rosa the fish to say thank you, Norman asks Bella about the bbq and wondered if she could set one up in the park at the weekend. The twins and especially Trevor are very keen on the idea, and Bella agrees to do it with the help of a certain fire station cook (a possible plot line there).
For Songs:
1.1: Norman's character song in Chemistry Set 9somewhere when Norman is sneaking up to play chemical tricks, maybe in the fire station kitchen), and Trevor's character song in Flat tyre (maybe while Dylis and the kids wait for trevor)
1.2 Sarah & James' character song somewhere in the first two acts
2.1 Dilys' character song in all in a good cause (maybe when she's snooping through Bella's trash pile), and Gently Bently in Bently's act.
2.2 Bella's character song in Bella's BBQ
That's what I think anyway. Who knows what actually happened.