The recommended way to tell whether a user is enrolled in Bill Pay would be to check the isEnrolled flag that comes in with billpay capabilities on the UUX platform. Here's a code example for how you would request that object through Tecton.

await this.tectonSDK.sources
  .requestPlatformData({
    route: `mobilews/refreshCapabilities`,
    method: "GET",
  })
  .then((response) => {
    this.billPayRights = response.response.data.isBillpayEnrolled;
    this.userId = response.response.data.userId;
    this.tectonSDK.setUserId(this.userId);
    return response.response.data.isBillpayEnrolled;
  });

As for unenrollment, the process for that is going to depend heavily on the Bill Pay vendor you're using. The endpoints you referenced do not support all vendors. I looked up the vendor for your FI (avoiding sharing any potentially sensitive information through this channel), and it does not appear that we currently have an endpoint implemented for programmatically unenrolling a user. This could potentially be implemented in the future. Creating a ticket through the dev portal with the details of what you're looking for would help get the ball rolling on that.