Although I think there are sufficient reasons to terminate the employee's employment, I am of the respectful opinion that your procedure could be challenged at the CCMA or other applicable forum (eg. Bargaining Council).
The "Warning" that you have attached does not state the offence clear enough. If it is a rule that only instructions by Property Manager and Owners are valid, the description of the offence should start with a description of the rule, and where it was stated in her employment contract. Thereafter the factual offence should be mentioned (as you have done) but it should be supplemented by a description of who Patricia Mhluali is, and why she does not have the authority to issue valid instructions, as well as specific reference to when she should have taken her leave, and when she actually took leave days (the dates).
The "Final Written Warning" document is contradictory. If you issue a Final Written Warning for Gross Insubordination, you are already disciplining the employee, and at a future disciplinary hearing the employee will be able to successfully raise the defence of double jeopardy. That means she can't be punished for the same offence twice. The document is also not describing the offence, but merely gives a definition of Gross Insubordination.
If you have already issued these documents to the employee, I'm afraid you will not be able to proceed with a disciplinary hearing unless she commits new acts of gross insubordination.
If you haven't issued these documents yet, then I suggest you issue her with a notice of suspension in which you set out the reasons for the suspension. I would also suggest that you issue a notice of disciplinary hearing simultaneously otherwise the chairperson of the disciplinary hearing may take issue with the fact that a long time period has passed between the date of the offence and the issueing of a notice of disciplinary hearing (but the time periods allowable would also be determined by your internal disciplinary code and procedure).
Should you have any further questions, do not hesitate to ask.