You ask some EXCELLENT questions, my friend! This same question could also be applied to the MERS Terms and Conditions and the MERSCorp Rules of Membership.
The answer to this quesiton is going to depend upon the law, particularly the Rules of Civil Procedure, of your jurisdiction. Moreover, the answer may also somewhat vary depending upon the stage of your foreclosure litigation.
In most places, authentication is accomplished by (a) a supporting affidavit, (b) an interrogatory response, (c) a response to a request for admission, (d) a response to a production request, (e) a response to a deposition or deposition on written questions, and/or (f) a stipulation.
Methods (b) through (e) involve discovery. Unfortunately, mortgage plaintiffs routinely engage in discovery abuse and cannot be relied upon to respond to discovery requests as the law requires. But you probably need to at least TRY.
A particular problem exists with the MERS Appellant's Brief because the Clerk of the Nebraska Supreme Court does not routinely certify documents. That is, you can write to the Clerk of the Supreme Court and obtain a copy of the Brief directly from the Clerk, which I would encourage you to do. But this will NOT be a certified copy.
The Clerk is (or was) otherwise VERY HELPFUL. The Clerk offered to send me a copy of the brief by oral request, simply enclosing an invoice. The cost of copies was then minimal. The Clerk sent me the copy the same date I orally requested it (the Clerk was adament that no written request was necessary).
Notwithstanding the friendliness and convenience of the Clerk, I would encourage you to request the copy by written letter, if time is available. Within the letter, expressly ASK FOR a certified copy of the Brief. Keep a copy of the letter. Consider having some OTHER extremely reliable and responsible non-party to the suit make the request on your behalf. This gets you an affidavit from a third party disinterested witness.
Hopefully, you will receive a written letter in response from the Clerk including an explanation that the Clerk doens't certify copies. You should also have the mailing envelope from the Clerk. Have the person making the request sign a proper, admissible affidavit certifying that the copy of the Brief was that received directly from the Clerk of the Nebraska Supreme Court, including the requesting letter, the response letter (if any), the invoice and the mailing envelope as additional supporting exhibits. Have the affidavit be extremely precise as to the circumstances and identification of each exhibit, describing each with some precision.
Even with such an affidavit, I would NOT rest upon my laurels nor would I stop with merely a supporting affidavit. I would expressly conduct discovery intended to authenticate the Brief.
This can be a little problematic. It is unfortunate to waste an interrogatory on such authentication. Also, the mortgage servicer may very well claim NOT to have any knowledge of the Brief and claim that they are unable to authenticate it. But this also somewhat immunizes you against an allegation that they doubt its authenticity and you have put them on notice of your intent to use the Brief as evidence well in advance of the summary judgment hearing or trial.
In some jurisdictions, such as Texas, interrogatories which ask solely for the authentication of documents do not count against the limit on interrogatories one may ask.
You can ask for an admission through a request for admissions that the copy is a genuine copy of the Brief MERS filed.
If MERS is a party to your litigation, you can ask these questions of MERS directly using written discovery. If MERS is a non-party, you can (a) name MERS in a third party complaint, serving MERS and bringing MERS in as a party (you would need a valid or at least viable theory to support this), or (b) serve MERS with a deposition on written questions asking for the authentication of the Brief.
The latter is probably the more efficient approach and could also be used to ask MERS other questions under oath about the Brief and its various allegations.
Similarly, you could take a deposition of a representative of MERS and ask for the production of the Brief in an accompanying subpoena. Then you could ask the MERS representative questions about the Brief directly.
I would recommend a combined approach anticipating that the mortgage investor or servicer will engage in illegal discovery abuse and that you may need to seek the Court's intervention to get the necessary proof. But if you have given the plaintiff the opportunity to authenticate the Brief by an interrogatory and/or request for admission, your attorney may be able to recover his or her fees in seeking the deposition where the plaintiff unlawfully resists discovery.
Finally, there is nothing wrong with asking the Court to take judicial notice of the copy(ies) of the Brief posted online or to take judicial notice of the Brief as it is filed with the Nebraska Supreme Court. Also putting in a copy of the published decision in the Nebraska case is also another means of enhancing the credibility of the Brief. It is undeniable that the Nebraska Supreme COurt decided this case.
In conclusion, also note that in a summary judgment setting the Courts in most jurisdictions are usually required to assume that all of the non-Movant's disputed evidence is true and to generally give the non-Movant benefit of the doubt. This may or may not apply to admissibility, so I would NOT assume this to be the case, but getting both the precise text of the Rules and the cases relating to the Rules would seem to be a good idea and being prepared to argue that you should be accorded this indulgence by the court is possibly also a useful backstop.
In the end, you need to ask a qualified attorney from your jurisdiction this same question and to do your own research and due diligence as to the Rules and the law of your jurisdiction. I simply describe a framework for further inquiry.
Best of luck!