Middle of last year I was retrenched from my job of 15 and a half years. I really battled to find another job. End of September last year I was offered a position for a family business. I was only given the contract to sign after working there a week. I was already not happy re certain issues for eg it is a permanent position but I am receiving an hourly rate so my pay varies greatly every month, especially if there are public holidays for which I receive no pay. Also I was told I would get less than the salary I wanted for the first few months but that at the end of February my salary would be reviewed to be more in line with what i was wanting. But as I could not find other employment at that time I accepted.

I was informed of my tasks during the interview but when I started working there it became apparent that I was not being delegated the tasks as per the interview, only the data capture. Nothing was said by the employers regarding this for the five months I have worked there but comments were passed in the office regarding the fact that i will only be responsible for data capture during conversations the employers had with other staff regarding their retirement and other conversations with fellow staff.
I became very bored and despondent as I finished the work quickly and always found myself having to ask for things to do.

On the first friday in March I was called into the employer's office and informed that they have done stock take and it is several thousands of Rands out and that this was entirely my fault due to some capture errors that they had photostated. the next day I received a handwritten warning informing 1) i am incompetent of my job 2) I am slow at everything I do apparently and 3) they had an issue with my phone usage. Also I was told when informed i was getting the warning that I ask stupid questions.
I have never received a warning regarding my work in my entire working life of 22 years now in fact have more often than not received praise/promotion /increases for my work even from usually difficult bosses. I was told I must decide over the weekend whether I was capable of doing the job for which I was appointed (the tasks of which were never delegated to me in the first place) or i must sign a contract as a data capturer only at a much lower rate (the rate was not discussed). This doesn't sound legal to me. If I have a contract stating my wage and job title (though no job description mentioned) surely this cannot be changed?

On the Monday I gave the employer the signed warning but didn't make any statements other than to apologize for the mistakes and say I will try harder etc. What followed was a hellish experience in which the employer's wife to whom I report directly delegating all the work that should have been delegated over the last 5 months all at once without little explanation on how to perform these tasks (company specific). I completed all the tasks to the best of my ability but had some questions about things I was unsure of but when i asked all the tasks were immediately taken away from me again as I was told/it was implied that I cannot use my own initiative and am again asking stupid questions. A witch hunt was launched and fault found with all my work and she spends her days next to me photostating "mistakes" that i have made for her file.

I was also informed that I am still within the 6 month probation period (also not mentioned in my contract)
Would i have any case against the employer in case of dismissal or demotion as I feel one of these two is imminent? or not? as they feel they can prove my incompetence for the job?
Please advise.