I have kept fancy goldfish for many years and I can totally sympathise with the SBD problems you have had with your fish.
Firstl I would say, don't beat yourself up about it - I, like you tried countless methods of treating SBD as well as preventative measures and almost all of my fish succumbed to SBD eventually.
I can honestly say that the issues I have had keeping fancies has begun to turn me off keeping them. Unfortunately because of years and years of in-breeding most fancies are genetically pre-disposed to SBD as well as other issues, even the very high quality specimens that cost hundreds of pounds can still have the same problems.
I consider myself to be a relatively experienced keeper of fancies and I used to be a member of the Goldfish Society Of Great Britain. Some of the breeders on there used to speak of show-winning fancies developing SBD problems, now if keepers of show winning fish can still experience problems then what chance do we have?
I have come to the conclusion that whilst fancies are an absolute joy to keep and very rewarding, the frustrations and distress of seeing a once perfectly healthy fish that you have kept and fed properly and looked after to the best of your ability become struck down by SBD vastly outweigh the positives of the hobby.
Sounds "do-able" however not many filters actually suck up solid particles of detritus, they deal with tiny floating particles fairly well and the biological filtration of course but most of the filters I use (which tend to be canister filters) do not suck up large-ish particles of dirt.
Usually this dirt is best removed during a water change when you can siphon the gravel.
Wow, that does seem a long time without seeing any drop in ammonia at all.
Having said that though I recently set up my new puffer tank and it took around 10 days before I saw a drop in ammonia and I used seeded filter media from my other tank to move things along so perhaps 14 days without any media is still considered early days.
I can?t see anything in your set-up or your technique to suggest you are doing anything wrong, my first thought was low PH but then you went on to say that you have raised the PH using coral so this is strange.
The only thing I do know is that starting a fishless cycle from nothing, i.e no seeded media can take a VERY long time to kick off, I?ve heard of some cycles taking 6 weeks or more (probably the first 3 weeks waiting for the ammonia to drop and nitrites to rise) which is why people always say grab filter media if you can as that can shave a good 3-4 weeks off - my cycle took 2 ? weeks from start to finish with seeded media.
As far as the fancies are concerned you are probably ok with 4 fancies as long as you don't overload the tank with decor - remember that although the tank is labelled as "300 litre" that is based on the tank being literally full to the brim without any decor, neither of which tend to be the case
Also a big factor with fancies is filtration, it's great giving them the space but you need to ensure your filter is up to the job too, what filter have you got for the tank?
No idea about compatibility between loaches / snails and goldies as my fancies don't have any other species in with them, best leave that for someone else to answer!
I've never liked them personally and have been put off by the noise the air pumps make, even the so-called "whisper" quiet pumps have an annoying hum to them.
Yes, it certainly sounds like your fish didn't like the air stone or pump much either! Sometimes the buzz from an air pump can reverberate into the water which can bother fish, doesn't happen often but it could be likely in your case.
In reality you should never need an air stone assuming you are using it to get oxygen into your tank rather than any decorative or aesthetic reason?
As long as you have enough movement on the surface of the water and the water is being broken enough at the surface that should be sufficient to get enough oxygen into your tank - I use my filters spraybar to agitate the surface and it works great.
My suggestion - do away with the air stone / extra tubing / pump and use your filter output instead, will save you hassle and a few extra watts in electricity!