Man, seems like that last Partner Showcase really did a number on the mood around here.
Nothing is a guarantee unless Nintendo themselves confirms it, but assuming development hasn't hit any significant snags, September looks good regardless of the amiibo wave given the general timing of character releases. Obviously things could be changed and be coming later, but there really isn't any reason to believe so right now.
For me personally, I see no reason to be doom and gloom about the chances of another character coming in September. Nintendo has typically been highlighting at least one announcement from them themselves per-month, and a given the gap we've had now with Min Min and operating on the assumption development is progressing nicely, September looks good IMO. Time will tell.