how possible is it for the US department of homeland and security citizenship and immigration services to send one an appointment and reschedul

It occasionally happens. Appointments are usually calendars at a service center, but there are occasionally glitches that require the local office to reschedule. The need to reschedule does have any bearing on whether the application will be approved or not. If you are concerned about the strength of your case, you may want to consider having a consultation with an attorney.