There was a dead snail? That definitely would have contributed.
I'd change the filter. Carbon when it gets full will begin releasing the contaminates, and there's been more than normal lately. I wouldn't risk it. A fresh filter would only help.
Off to bed now, but let us know what the ammonia levels are after tomorrow's small wc. (remember, no big siphon or disturbing the substrate/plant roots. sorry to be redundant, I just don't want any more setbacks for you!)
Using the airline tubing is ok to get at the rest of the exposed mulm, and removing any dead plant leaves is good too. Hopefully no more dead snails, or fish.
Those little tetras deserve some frozen brine shrimp treat after all this gets resolved.