I think you are doing wonderfully, and it is likely just as traumatic for you as Light.That is also the question we had (later after we got home). The vet never really explained the whole picture. He said that the food paste (link) that we tube feed her will be good at keep flushing stuff down her track since it has a good consistency, in addition to a nutrient balanced meal.
He also said that she can eat other stuff if she wants to.
It seems that he thinks tube feeding is easy and did not think that our newbies can have a little trouble at first.
If the fluid was coming back up her eosophagus then it is likely that her crop was full, Are you feeling to see how full it is before feeding? Maybe cut the amt in half if she is eating other items.
You're both doing an amazing job with her, not sure I would be so patient or calm!