Honestly right off the bat it does not sound like liver failure. As someone had said, the ALT is not terribly high. I have seen it in the thousands. One of my pups was around 3000 at one time and she is fine now.
I am assuming they are going to to bile acid testing. All I can say is that yes, a single external shunt is operable but I do understand your concerns about her having surgery. But, you know they all must have surgery at some point because most of them need dentals. Seisures alone should not prevent anesthetic procedures...just speak with your vet regarding this. I have seen some pretty old and sickly pups go through anesthesia without issues. BUT...take it a step at a time.
As for liver shunt showing up later...that often happens. Matter of fact it can happen without any noticable symptoms OR alterations in blood work. My Teddy Bear is living proof of that...here is a link to his story about his shunt that was not picked up until he was around 8 years of age:
http://www.yorkietalk.com/forums/sic...ver-shunt.html
Speak with your vet about feeding liver diet...either Royal Canin or Hill's. I did that with one of my fosters who we thought might have a shunt and the change in her was amazing. She ended up diagnosed with MVD .. not a shunt but remains on the Hill's LD and is doing well.
I personally would stop the Beneful since there are some questions about it.
It could end up that her seizures are not related to liver at all. Time will tell. I hope that the meds are keeping her fairly free of seizure activity.