It depends if the Court grants the Amicus in the case. Given the facts, it sounds like you really do need one so that you can see your child. Also, if your ex isn't abiding by the agreed order, then there is an contempt of court issue. You must file a motion for Contempt of Court so that your ex realizes the seriousness of not adbiding by a court order - especially in Harris County.
I highly suggest seeking an attorney.
This is just an opinion, and is no way to take the place of an attorney that knows all the facts of the case. Therefore, I would seek such advice as soon as possible.
If the Mediation Agreement has not been turned into an Order of the Court, it should be soon. Then you could file a Motion for Enforcement of that Order to enforce the visitation schedule that you and your ex agreed upon in mediation. I tend to agree that an Amicus Attorney does sound appropriate for you case. If your ex makes significantly more money than you do, then I would try to make this clear to the Judge so that the Judge will consider ordering your ex to pay for the Amicus.
This answer is for informational and educational purposes only. This is not intended to be legal advice and an attorney-client relationship has not been formed. Please contact an attorney of your choice with whom you can discuss all the relevant facts and seek legal counsel and advice regarding your legal issue(s).