I'd strongly advise against a hang on filter with Goldfish.
Apple snails prefer warm water, they tend to slow down in cold water tanks. I'd consider whether it's worth adding them and creating the extra waste. They are likely to spend much of their time closed up in their shell.
You could try taking them out and using hydrogen peroxide to clean them, use a cotton bud or an unused toothbrush to clean the holes. Cotton buds designed for cosmetic use have pointed tips and would fit into even smaller holes than standard ones.
A toothbrush would work in the tank too if you don't want to take the rocks out.
Re: Advice - how do you know when it is definitely 'time'
It sounds like you are doing everything right. I'd suggest trying to reduce the nitrite and nitrate a little bit more. Curling can be caused by high nitrates. I have a comet that has developed an over sensitivity to nitrate, she can barely take 40ppm before starting to bend. It might be worth trying an actual salt bath too, the fish might respond more to a high concentrate of salt.
If you want to try the bath you'll need a container that can hold at least 4 litres of water. Then little over a tablespoon of salt dissolved in hot water. Bath the fish for about 3 to 4 minutes.
Ideally, take fcmf's advice first and wait but I recommend the bath before euthanising him.
I have many used fountain/pond pumps. Various sizes, they work fine but may be a little slower than a brand new pump. Could be useful for someone wanting back up pumps or on a low budget. All I'm asking is you cover the postage (unless you live close enough to pick up). The pumps are either Blagdon or Libel. They may not have their original fountain attachments, hence them being good as spares.
I still also have a sealed Libel Xtra 1600 which I'm selling for ?10 plus postage.
I used a calculator that is on the INJAF website. It says I need 31g worth of plants for this tank. You're right though, it's ultimately trial and error.
In other news. Winston's tank has snails in it I don't know what to do about that. I'm hoping the fish in the other tanks dealt with their snails. I've heard of goldfish eating snails. Winston and Billy are not up for anything that radical though, they prefer the more slow paced lifestyle. I could get an assassin snail, I'm not crazy about the idea of killing anything.
I also added a couple of baby cryptocoryne. I have more of both plants and I intend to add another three cryptocoryne. A bundle of the Elodea is floating because Darren has been pulling it up, as well as using it as a snack. Samantha doesn't seem to like having it the tank and is spending most of her time on the bare side.
Re: How to keep goldfish whilst pond liner being replaced.
Yes you will need a pump and some cycled media ideally, otherwise you'll need to do big daily water changes, especially if you kept them in an 84 litre storage box. I'd shop around and see if you can buy a bigger storage box, I've seen Wham boxes that can hold 160 litres. You could split the fish and keep the smaller ones in the 84 litre and the bigger ones in the larger box.
As for filters, you could check ebay or see if you have a local selling page on Facebook. You could probably pick a couple up at a low cost. Another option is to improvise. If you have small pond pumps or fountain pumps these can be temporarily used as filters and oxygenators. You can wrap sponge around the intake of the pump. I'm not familiar with pond filters but if it's possible to take some of the media and use it around the temporary pump, the pumps will build a cycle.