Hola Aleksandre

In reference to your first question Which action is (not) completed in the past?:

It is the action of "there being food" , so we imagine this as the existence of food in the party without necessarily seeing the end of this (we don't see the food being gone) - We are simply describing part of a scene at the party. This is why we are using "había".

In reference to your second question What if we add "ayer" at the end of the examples?

Generally, with a time frame like ayer we use the preterite because we normally talk about actions that happened in a frame that is no longer connected with now (when we talk, present time) and that means, using the preterite, hubo, tuve, hice... seeing these actions as things that happened yesterday and we see the end of it, completed actions. 

However, we can still use if we want the imperfect with the adverb ayer - if we do that, we place ourselves in yesterday and the actions, e.g. ayer había mucho tráfico, ayer llovía mucho, ayer estábamos aburridos... are seen as actions that again were happening during the previous day but the action is seen in our minds as something happening during the previous day "as they were happening", not as something that had an end.

As a contrast we could have these two (equally correct):

1. Ayer había mucha niebla. 

2. Ayer hubo mucha niebla.

1 is saying that there was fog yesterday, during the day, without specifying that it started and finished at a certain point so we see this as an "open" action.

2 is saying that there was fog yesterday, and this fog had a beginning and an end; it could be implying that at some point yesterday (maybe for a couple of hours) there was fog and then it stopped being foggy. Or it could imply that there was fog all day yesterday and there was an end to it at the end of that day; therefore that is seen as a completed action, justifying the preterite.

This is a concept that we are aware is quite tricky for non-Spanish speakers because the differentiation of these two tenses might not exist in other languages. I always use the example of "abrió la puerta" and "abría la puerta", the first is illustrated by a door completely open so it shows how the action of opening the door is completed; the second is illustrated by a door that is only half open or opened without being completely opened, showing that it is an action in progress. This is how we visualize these two actions, one of each using a different past tense.

Saludos

Inma