Based on the information you provided it does not sound like water quality is the issue unless you are dosing something you didn't mention or used some unsafe silcone for repair,etc.Nitrates even when pretty high won't harm fish. If your sg is 1.026 you alk is likely fine and fish don't care much about that either in usual ranges.
8 weeks without fish will usually clear cryptocaryon from a tank although one study showed cysts retaining vialbility for 72 days.
Not sure from the post how you cleared up the ich on the two fish.Depending on the rigor of the treatment process , some may have remained in the nose, mouth, or gills and reinfected the tank.
Secondary infections, bacteria, amyloodinium(velvet) are common after an ich infestation. It's also possible but unlikely that the treated fish contracted brooklynella .