Aside from the manuscript evidence, which seems inconclusive, the most practical reading is to take it exactly as it is. Or as a not in the NET Bible says in a note on v. 8...
"Jesus may simply have been refusing to accompany his brothers with
the rest of the group of pilgrims, preferring to travel separately and
“in secret” (v. 10) with his disciples."
Biblical Studies Press, The NET Bible First Edition; Bible. English.
NET Bible.; The NET Bible (Biblical Studies Press, 2006).
Whether the original author included "yet" or not it seems like he meant for readers to understand that Jesus wanted to go later secretly. Perhaps we need to re-evaluate what we consider deception. It is one thing to purposely mislead someone so you can take advantage of them. It is another thing to keep you own counsel. Jesus did not owe it to his followers to let them know his future plans. All they needed to know at the moment is that they should go ahead and go without waiting for him. He wasn't going. He didn't say, "I will never go."
Just because Jesus didn't reveal his intentions doesn't mean it wasn't technically true that he wasn't (at the moment) going with the disciples. Semantically, it likely feels worse in English. Including "yet" might even have been an appropriate translation choice if the original copyist was familiar with the underlying Aramaic.
In Genesis 2:17 God says of the Tree, "you shall not eat, for in the day that you eat from it you will surely die."
Whoever first wrote that down knew the rest of the story. They knew God didn't kill Eve on that very day. They also didn't go back and edit it to say "you will eventually die."
In the same way, it is hard to imagine that John is trying to portray Jesus as a liar. Jesus was unpredictable. A characteristic John found all the more attractive. "Jesus told us he wasn't coming, which was disappointing to hear; but then he showed up anyway! It was amazing." If it were immoral deception it seems the disciples would have made a bigger issue of it in a negative sense. On the contrary, the event is reported with awe. In fact Jesus, who could have remained in secret and avoided questioning, boldly stood up and revealed himself even when other were afraid to mention his name.