1. As is showing that two things are happening at once. Emma is watching, passengers are boarding, and goods are loading all in the same moment of time 2. A synonym for as in this context could be "while"- meaning at the same time.

I think the word “as” lets you know that this action was not a singular event that was completed, but rather that Emma was spending a good amount of time on the deck, watching the passengers. I think the best translation might be “mientras”. Emma watched as passengers came on board… Is similar to: Emma watched the passengers as they came aboard… (I’m not sure about the accuracy of this translation, but it might give you a better sense of the meaning: “Emma miraba los pasajeros mientras subían a bordo”) The sentences below might be used if the author wanted to express that Emma did some thing and then the moment passed, or just to make the moment sound less relaxed: Emma watched passengers come on board… Emma watched the passengers come on board… Emma watched the passengers board the ship…

"As" is a conjunction. Actually, the sentence contains two conjunctions: "as" and "and" which are used to join three independent phrases. Each phrase is a complete sentence all by itself: "Standing on deck, Emma watched." "Passengers came on board" "Goods for trade were loaded, including livestock". You can replace "as" and "and" with any other conjunctions ("but", "although", "however", "because", "while", etc.) and your sentence still makes sense. However, to keep the same meaning you would replace "as" with "while".