Q: Why do you follow Umar when he changed the religious law. For example, he banned Muta. How can you follow a man that changes religion?
A: Muta is a term that refers to marrying a woman for a specified time period. For example, if a person came to your sister, mother, etc. and said the following: John hereby gives her one dollar for three minutes of adult gratification, would you accept? Naturally, most people would refuse accommodating such a situation. Religion is no different.
Likewise, the prophet (SAW) did not allow Muta to continue and prohibited its practice.
Sayyidinah Umar (RA) only assisted in enforcing the various laws. Sure, many laws Sayyidinah Umar preferred to enforce and had a preference towards. This and other laws only become law for Muslims when the prophet (SAW) himself through revelation caused for its acceptance. Other laws become law only as a result of consultation between the Sahaba (RA), not on a mere whim. It would also be fair to say that the Sahaba understood religion better than our contemporaries.
Thus, your assumption that Umar (RA) prohibited Muta without the approval of religion or its consensus is not correct, he (RA) only assisted in its enforcement, no different from the other khaleafs, etc. on this matter.
Allah Certainly Knows Best.