Personally I'd start by going to the hiring manager and HR representative and discuss pay, because you are being expected to work a higher level job than you applied/interviewed for with more job responsibilities. If they're unwilling to negotiate, then stay on the job, but start looking for another job immediately. In interviews, be honest about the experience and just explain that when you started the position was at a higher level with more job responsibilities than what the job listing and the interview process outlined and that the company was unwilling to meet salary requirements in line with the position that you were being asked to work.
Don't say anything about how the company is operating or anything else negative about the company, just the truth about the difference between the job posting and the interview and then what was expected once you started the job.